OSR: [Creature] gains +1 HD every time it...

I like the idea of conditional creatures. Here are 10 creatures that gain HD (Hit Dice, corresponding to HP, attack, saves, etc.), prompted by G+ people.

...Eats a Condor.

-Onno Tasler

The Bird-Eating Stump-Toad
HD: 3
Appearance: a fat grey-brown toad the size of a cow with strangely human limbs and small, cruel eyes.
Wants: to eat condors, to remain unharmed.
Armor: as leather
Move: normal, can hop 10'
Morale: 6
Damage: 1d6 bite, swallowed on a 6.

It wants to eat condors. It wants to eat condors so badly. It licks its hideous warty lips whenever it sees them circling overhead. Their nests are in cliffs it cannot climb. The toad will pay anyone for fresh condors, dead or alive. It eats them messily. It can provide secret information (the toad can hear very, very well) and dig tunnels. It will not serve; it will demand to be served and deign to offer a reward. If The Bird-Eating Stump-Toad reaches 6 HD, it splits in two. The new toads will target the species it sees first (condors, humans, etc.)

...Hears Profanity

-Brian Ashford

The Decorous Sisterhood
HD 1
Appearance: a stern nun in a black and white habit. Wields a wooden switch.
Wants: to serve the Authority, maintain moral order, and strike the wicked.
Armour: as plate+shield. Also, are you really going to hit a nun?
Move: normal, but can glide (as a charge attack over terrain).
Morale: 12
Damage: 1 HP on a hit, but target must Save with a -4 penalty or be Stunned for 1 round. It also really stings. Targets cannot be reduced below 0 HP, but at 0 HP, are automatically stunned and can do nothing but yell.

A single Decorous Sister is usually found maintaining a shrine or an orphanage. She will appear completely harmless and may enlist the PCs to help with a simple quest. Should any of them swear, she will strike them with her wooden switch (automatically hits) and repremand them. Should they continue (likely, given that a nun just hit them with a wooden switch), she will hit them again and summon a second Sister (a 1 HD creature). This process could continue indefinitely until the PCs have been swatted into submission.

...Gets Hit.

-Brian Ashford

Corpulent Callowfex
HD 2
Appearance: a morbidly obese velociraptor with hairy moles. A diseased lizard with tufts of grey hair.
Wants: to bite things
Move: 1/2 normal
Morale: 8
Damage: 1d6 bite, 1d6 claw.

A backwards castoff from an orthogonal timeline. Don't worry about it too much. Nobody understands time travel anyway. It needs to die to live. Every attack that hits it give the Corpulent Callowfex +1 HD instead of dealing damage. At 4 HD, it begins to look much more sleek and cunning. At 6 HD, it gains 2 additional claw attacks and can move at normal speed. At 8 HD, it will begin to speak, cast spells, and act like the renewed and triumphant apex predator it truly is/was/will one day be. You should probably run. Or tie it up - anything that isn't an attack doesn't trigger the HD gain.

...Has its Life Spared After Being Defeated by the Party.

-Iacopo Maffi

Craven Stalker
HD 1
Appearance: a thin, bony humanoid with huge bulbous eyes and long bent fingers. If it weren't for its needle teeth it would be almost pitable.
Wants: to feast on the hearts of the PCs.
Move: normal, but can move through shadows (as an instant teleport). It will not reveal this ability.
Morale: 12 (appears lower)
Damage: 1d6 bite.

It's not a creature. It's a curse. Someone called this thing into being and sent it after the party. It takes  lot of blood and a lot of time to call a Craven Stalker into being. It will hunt the PCs and attack in a ridiculously inopportune way. One or two attacks will cause it to beg to surrender. It will promise treasure, riches, fame.

It will lead the PCs to these things and to incidental danger, danger the PCs are certain the Craven Stalker could not have forseen. It was an accident. A tragic mistake. Their own incompetence or greed. It will do this two more times, gaining +1 HD each time. Poor creature; it doesn't know any better, the PCs think.

When it reaches 3 HD, the Craven Stalker begins laying Suspicions. Each Suspicion is a tiny red egg dropped into the ear of a sleeping PC. It grows slowly, driving the PCs apart or causing them to murder each other. The Craven Stalker will feast on the remains - particularly the hearts. It will secretly, stealthily, hound the remaining PCs, introducing more and more Suspicions until the PC lives an isolated life of pure paranoia, dies of a heart attack, or takes their own life.

...Collects a Molar.

-Chris Wilson

Tooth Fairy
HD 1
Appearance: tiny humanoid with a large head and gossamer wings.
Wants: teeth.
Move: 2x normal, flying.
Morale: 4
Damage: 1 (prod) or remove 1 tooth.

Tooth Fairies are harmless, mostly. They lay their eggs in teeth, producing a 1 HD new Tooth Fairy in 1 week. They can be bottled, trained, or tricked. They aren't very bright. Boneyards are infested with the things.

...Spends a Day in the Sunshine

-Gregory Blair

The Hidden Filth
HD: 5
Appearance: a squamous black mass of tar, eyes, and tentacles.
Wants: everything
Move: 1/4 normal
Morale: 12
Damage: 1d4 bludgeoning per round per HD

The PCs shouldn't have unlocked that ancient tomb. They shouldn't have broken those primordial seals. But they did, because they were shiny, and they went back to town to sell the metal and get drunk. And while they were gone the Hidden Filth slithered out.

It was trapped underground long ago. It drinks sunlight like a fire drinks gasoline. Plants? Amateurs. The Hidden Filth grows and spreads. It will slither into some secluded but sunny place and begin to grow. Every day, it gets a little bigger. Each day, it gets a little harder to kill.

...Has One of Its Parasites Cut Off.

-Steven De Waele

Parasite Dragon
HD 3
Appearance: a rail-thin dragon, dry, flaking, with pulsing black veins and mad rolling eyes. Each limb is coated in a single coiled parasite, a fat lamprey-like slug-fish-worm with tiny grasping spider legs. Two worms coil around its torso like a lover's arms. Another slides along the tail. A final worm wraps around the dragon's scrawny neck and burrows into its chin, hanging down like a goiter.
Wants: blood, magic, magical blood
Move: normal.
Morale: 8
Damage: Each parasite (there are 8 total) spits 1d6 acid damage in a 20' cone if threatened. Can also bludgeon for 1d8 damage. Other attacks: as a dragon. Breath is only phlem until the dragon reaches 6 HD.

Killing the dragon without killing the parasites is difficult. They want to keep their host alive. The parasites are as wise as any human but are completely remorseless and eternally hungry. Cutting them free will make getting to the dragon's flesh easier, but each dead parasite restores 1 HD to the dragon, and the dragon is completely and murderously insane.

...Is Mistaken For the Mayor.

-Peter Webb

Dopplepollster
HD 1
Appearance: the Mayor, of course!  Yes, I remember now... but didn't the Mayor have blue eyes and not... egg-yolk yellow?
Wants: adulation, proof that humans are idiots
Move: normal (strides)
Morale: 6
Damage: 1d4 bludgeoning. Die size, number of attacks increase as HD increases.

Hello Citizen! Why yes, I am most definitely the Mayor! Observe my many shapely limbs. Truly, I am a good mayor, who passes may bills, and not a creation of a secret underground race at all. No, definitely. I deny this. I have no recollection of being constructed. None. My birth was a normal human birth in this VERY TOWN yes. Present your infants to me. Yes. I am the Mayor.

The Dopplepollster is unconvincing. It feeds on contempt. Its creators are certain that humans are idiots. Every time someone mistakes the Dopplepollster for Mayor, it gains 1 HD. When it reaches 10 HD it will go on a murder spree (stats as something horrifically murderous) while screaming "YOU FOOLS, I AM NOT THE MAYOR." It will then repeat the process in the next en-mayored town or city, starting over at 1 HD, unless killed. It usually starts at festivals, balls, or events where killing it immediately would cause a scandal.

...Is Lied To.

-Alex Chalk

Veritasphinx
HD 5
Appearance: a green-silver sphinx, meticulously clean, with a beak like a parrot and eyes like sunken nuclear reactors. It purrs in 3/4 time.
Wants: only the truth.
Move: 2x normal
Morale: 9
Damage: 1d8 bite/1d6 claw/1d6 claw, or eye-wither (50' range, 2d6 damage, Save or gain a stutter for 1d10 days).

The veritasphinx lives in libraries, archives, and research facilities, sometimes against the will of the owners. It is difficult to remove. It will pick a vital passageway and guard it. Anyone passing by must state their intentions truthfully. If they lie in any way, the Veritasphinx shouts "LIE" and then licks her paws. She wants the whole truth; omission will provoke follow-up questions. She can sense embarrassment through 10" of lead.

For every lie, a 1 HD parrot-kitten pops into existence just out of sight. When the liar leaves, if they are unrepentant or troublesome, the swarm of kitten-parrots will descend and devour them. Half the kitten-parrots only tell lies; half only tell the truth. They can be tamed with math.

...Eats a Wizard.

-Luka Rejec

Wizard HydraHD 3+1d6
Appearance: a horrible shapeless mass of dry skin with oddly jointed necks and fat paddle feet. It has HD heads. Each head carries the distorted face of a wizard it has devoured with its enormous lower mouth located on its torso. The heads moan and wail; the blind torso-mouth only dribbles.
Wants: to eat wizards
Move: normal
Morale: 12
Damage: 1d6 bite (swallowed on a 6, if swallowed, Save or die.)

What horrible accident spawned the wizard hydra? Who will save us from this menace? Each head casts the spells it could cast in life: rays, beams, healing. It lurches and gurgles, blasting everything it sees and devouring any wizards it can find. Every wizard devoured becomes a new head; every head brings fresh terror. Roll randomly for the spells or use the Beholder spell list.