We've all read the Stupid Monsters someone was paid to make article, haven't we? Go do so if you haven't. Don't forget to check part II.
Anyway, "silly monsters" are, in my opinion, as much a part of D&D lore as Greyhawk and Beholders. D&D wouldn't be the same without Owlbears, Mimics, Leucrottas and their ilk.
Weird monsters seem to be made up of one third "Two animals mashed together", one third "creatures mimicking loot or common dungeon features" and one third random other stuff (flail snails and flumphs).
Item mimickers are easy to come up with. Roll up some random treasure and make up a justification to how a creature is masquerading as that item. That generator likes to generate coins and gems, so just make up little flesh-burrowing bugs that looks like coins or gems and proceed to skip those entries in the future. (The coin golem is another classic.)
Random animal mashups could be a little harder. One might be tempted to just modify existing mashups - resulting in hawkbears and dragon-winged manticores. In a fit of inspiration, I compiled a list of random animals to roll on (mainly taken from the d20 SRD).
25 Manta Ray/Stingray
So bring out your d45:s and roll a pair (Invisible Castle might help) to generate endless kooky monsters (or technically 45^2 monsters, which is 2025). Let me try it for you:
- Weasel + ape = The APEWEASEL, a gaunt apelike beast with a surprisingly flexible body and a nasty bite. (Not that I'd want a gorilla to bite me.)
- Donkey + snake = The SNAKEY. Looks like a regular donkey at first glance, and does indeed use this appearance to attract predators, but has a ridiculously poisonous bite and kicks just as hard as a regular donkey.
- Hawk + lion = The LIONHAWK. Looks a lot like a manticore, but without the spiky tail.
- Elephant + manta ray = The rogue ELEPHANTA - roughly as large as a pony - glides silently along the steppe on its wing-fins, ready to spear unwary travellers on its tusks.
- Boar + spider = The SPOAR, an eight-legged tusked monstrosity, prowling the woods with the patience and stealth of a web-spinning spider and the fearlessness of a boar.
- Hyena + snake = The HYENADDER. A creation of the demon lord Yeenoghu, this fell beast has a hyena's build, but the skin texture and poisonous fangs of an adder.
- Stingray + octopus = The OCTOSTINGER has not one, but eight stingers at the tips of its tentacles. Divers beware.
- Wolverine + rhinoceros = The WOLVERHINO won't stop goring you with its horn until it hears bones crack.
- Lion + manta ray = The LANTA is an aquatic version of the lionhawk. It preys on coastal settlements and fishing boats.
- Deer + elephant = The DEEREPHANT just has a lot of sharp stuff on its head. Seriously, what do you expect me to do with that?
There you go, ten random monsters. Bet you a dollar some of them already existed.