"Hippopotomonstrosesquippedaliophobia"
Hippopotamus does mean river horse and I was caught out by that (note the o instead of a in ...poto...). I think that word is really a joke - lol - a bit like floccinausilihilipilification, which I wont bother looking up the speling 4.
Various sources suggest it's a literal joke, e.g. https://en.wiktionary.org/wiki/hippopotomonstrosesquipedalio...
I was gonna say, you spelled that wrong :p