Had to vote for Displacer Kitten, since one actually appeared in a game I was in a while back.
To preface, the day before, we'd fought Displacer beasts in a vineyard, and the Rogue had been very nearly killed by them. That night, the Rogue is searching the study of the noble who owned the vineyard for evidence, when she hears a scratching at the door, and a tentacle snakes under it. Freaks and bugs out without the evidence we needed, gets back & tells the party about it, and finally realizes how /small/ the displacer beast tentacle was, and that it could only have been a 'kitten.'
Same DM took an undead monster called a Charnel Hound, which was large. We had these perfect plastic minis for it (Zombie Dogs from the Zombies! board game, and a lot of 'em), but to scale, they'd be average sized dogs. So she dialed it down to minion status and called them Charnel Puppies.