(This relates to the 1966 Batman movie, but only in a tangential manner before moving on to other topics, and it should make sense even if you’ve never watched that.)
I happen to have watched the 1966 Batman movie recently; I’d not consider it particularly good, but at times it’s amusing. Nevertheless, consider the following exchanges paraphrased from said movie:
Police officer, reading riddle: “‘What weighs six ounces, lives in a tree and is very dangerous?’”
Robin: “…A sparrow with a machine gun.”
Batman: “Precisely, Robin! The only possible answer.”
Later, and more plausibly:
Batman, reading riddle: “‘What goes up white… and comes down yellow and white?’”
Robin: “…an egg!”
Riddles are necessarily somewhat vague, and I think that they usually don’t (and can’t be expected to) unambiguously identify their answers out of the vast range of concepts that one could reply with – and they don’t need to be that exact. There exists an unspoken assumption that the answer of a riddle is a reasonably ordinary concept in the culture and context in which the riddle is asked. The challenge of a riddle is not to recall or devise some exotic entity fitting the description, but to find the connection between a strange-seeming set of qualities and an object that is in it’s own right quite ordinary.
I take this to mean that, within the private culture of antagonism that has arisen between the Riddler and Batman, the idea of a sparrow with a machine gun has become reasonably commonplace.
Of course, this could be played for humor in the reverse manner – “What goes up white, and comes down yellow and white, is of course a motion sensitive color-changing baseball, a recent invention of my own. The answer seemed quite obvious to me.”
(It can be interesting to imagine odd entities like this – what sort of strange monster is Alive without breath, As cold as death; Never thirsty, ever drinking, All in mail never clinking?
Likewise, a four-two-three-legs creature wouldn’t seem so out of place in some medieval bestiary, which is of course why that riddle works; if you don’t know the answer already you may find yourself mislead by the attempt to recall a creature with that property.)
Perhaps a devious sphinx says “What walks on four legs in the morning, three in the afternoon, and four again in the evening, except on weekends, when it generally sits down for most of the evening, and Tuesdays when it walks on four legs at all times of day?”
The answer? “Me!” says the devious sphinx. “Let it not be said that I lack dedication. I’ve spent the last year walking strangely in order to set up this riddle. If you didn’t think to investigate my habits before confronting me, you’re not paranoid enough!”