All of this is useful and important information, and it's cool how things get to be named things that they aren't, but the linguistic point I wanted to make is in relation to the last one.KILLER WHALES aren’t whales. SLOWWORMS aren’t worms. HORNY TOADS aren’t toads. BEARCATS aren’t cats. (Or bears.) pic.twitter.com/23MSQZqDjd— Haggard Hawks Words (@HaggardHawks) August 23, 2016
Each of these has two parts to its name, either an adjective plus a noun (slow+worm, horny+toad) or two nouns (killer+whale, bear+cat). (I've analysed 'killer' as a noun here, but I suppose it might be an adjective too - it doesn't much matter.) In each case, it's the second part that tells you the type of thing that it is (or isn't): a worm, a toad, a whale, a cat. That's because when you put two things together in language, you (nearly) always have one that's the 'head' and the other part modifies the head in some way. Here, it tells you what type of worm/toad/whale it is: a slow one, a horny one, a killer one. This is a general fact of English: the Right-Hand Head Rule.
This means that even when the two parts of a compound are nouns and either could theoretically be the head (a bear cat could be a type of cat or a type of bear), we interpret the right-hand element as the head. This is evident from the wording of the tweet, where we're told they're not a type of cat, and the fact that they're also not bears is added as a humorous parenthetical, just in case we were tempted to think that.