Non-D&D or at least non-MM Monsters

[Dannyalcatraz]

the velocihamster was a variant Giant Space Hamster (remember them?)

Lordy, yes!

that was built a lot like a velociraptor. If you think that's dumb,

No, I don't!

the same adventure had the mighty massive tinker-gnome built Mechahamster.

AWESOME!

 

[Stormonu]

One book of three for me (though I haven't finished the 3rd)

Perhaps my favorite was the Baetle (yes, purposely "mis"spelled), which was based on the beetle-like minion monsters from Dark Crystal, though mine could also spit acid.