Fascinatingly, at least to me, Yiddish has two words for “a rabbit:” krolik (קראליק) and kinigl (קיניגל), the first being more common than the second. Krolik derives from a Slavic root, as in Polish “król, królik,” while kinigl is clearly Germanic in origin. The question is, what do rabbits have to do with kings? The first step in this mystery may be that Polish “król” is also the word for “king” in that language, so it could be that Yiddish carried the king-rabbit connection over from Polish. But that begs the next question: why did the Poles connect kings and rabbits? To that, I have no answer at present…
In other words, both Polish and Yiddish add a diminutive suffix (-ik and -l) to “king” and so downsize him to a mere rabbit. But neither modern German nor Russian seem to follow this rule.
In the Russian case, the Polish word królik was directly borrowed as krolik – with only a change in the stressed vowel – which still feels like a diminutive but of nothing in particular. The Russian word for king, korol’, becomes korolyok when a similar suffix is added – a word that means a lot of different things (e.g., a bird or a persimmon cultivar) but none them is a rabbit.
The answer can be found in older German: see the archives of the Mendele mailing list at Columbia (Vol. 16.003, June 2006). Look for the messages from Joachim Martillo and Bob Rothstein. In particular, Joachim Martillo explains:
The Yiddish form is diminutive, in imitation of Slavic, which has a loan translation of G. Koenig ‘king’ with the Slavic diminutive suffix…
Bob Rothstein writes:
It [Polish “królik”] derives from an old German name for the rabbit, “Kueniklin” or “Kueniglin” (from Latin “cuniculus,” which also gave the Yiddish term). The German word was misunderstood as a diminutive of the word for king (“Koenig”), and so the Poles called their rabbits “little kings” – and the Russians borrowed the Polish mistake in the form of “krolik.”
Cuniculus, mhd. künigel; nhd. Zusammensetzungen Künighase und Hasenkünlein.
Which means, more or less:
Latin cuniculus > Middle High German künigel; New High German compound words Künighase and Hasenkünlein.
Wackernagel’s book is a treasure.