ok so it's an English language fuckup thing.
Eh... it's a human organization and thought process thing. We like things to go in buckets, we have a hard time when the bucket changes, or when a thing goes in more than one bucket, especially if that bucket is shared with things that make no sense. The platypus doesn't seem to mind what buckets it gets put into but it causes no end of concertation amongst humans.
It's also specifically a problem with the 'stickiness' of language. Calling a thing that thing long, long past the point that we've generally accepted it isn't that thing anymore is a real problem.
Examples (most of these exist as the sole species of their family):
Electric eel, not an eel.
red panda, not a panda.
king cobra, not a cobra (they are kings though).
mountain goat, not a goat.
honey badger, not a badger.
mantis shrimp, not a mantis nor a shrimp.
American buffalo at least have a second, appropriate name. They aren't buffalo though.
flying lemur: doesn't fly, nor is it a lemur.
maned wolf: not a wolf.
horned toads: nope, not a toad.
koala bear: newp, not a bear.
virtually all fish: not whatever they're called.
special mention: mountain chicken. It's a frog.