Literally: “test rabbit” / meaning: “guinea pig” (for an experiment)
This term is used quite loosely and frequently in German. For instance, if you try a new recipe on a friend—before you might embarrass yourself in front of your date—he is your Versuchskaninchen, not your “guinea pig”.
As there is not much interesting etymological stuff to unfold here, let’s focus on another question: where the hell has the guinea pig its name from when it clearly originates from South America and is not even remotely related to a pig? Well, the latter part maybe has something to do with their squeaking noises that are said to resemble piglets. As for the first part, one theory is that they used to be sold for a guinea, which was an old British coin. The other has to do with the fact that “Guinea” was used to refer to a far-off, exotic country no one knew a lot about—so maybe this generic term was enough to label the animal as something foreign.
Back to topic: The English version beats the German expression in terms of accuracy. According to US Department of Agriculture, there were 171,406 guinea pigs used in American laboratories vs. 133,634 rabbits. These figures are not something humanity should be particularly proud of.