All right, so let's try to deepen our understanding of the words "hippopotamus" & "hippocampus", in particular evaluate your feeling these are misnomers because they have nothing to do with horses (as emphasised above).
I googled origin of hippopotamus, and I've got this:
Dictionary result for hippopotamus
/ˌhɪpəˈpɒtəməs/
noun
noun:
hippopotamus; plural noun:
hippopotami; plural noun:
hippopotamuses; noun:
pygmy hippopotamus; plural noun:
pygmy hippopotami
- a large thick-skinned semiaquatic African mammal, with massive jaws and large tusks.
Origin
View attachment 2042
Middle English: via Latin from Greek
hippopotamos, earlier
hippos ho potamios ‘river horse’ (from
hippos ‘horse’,
potamos ‘river’).
So, hippopotamus was named as such in middle-centuryEnglishmen who knew Greek (or even Greeks themselves) who thought of this unusual animal as "river horse". So, here is the "horse connection": the early explorers to have seen the animal, misunderstood it and invented a simplistic, silly name. Certainly, if we asked native African tribes what the call hippos, I am sure they would call it totally different than zebras/horses. Sadly, we ignored their knowledge and the name invented by ignorant travellers stuck. There are countless similar examples. E.g. I've already mentioned, I have a problem with the name "Australian magpie" for a bird belonging to butcherbird family, that has nothing to do with magpie, but the name stuck and it's hard to undo it now.
Re hippocampus.
I found the origin of the word described here:
https://www.medicalnewstoday.com/articles/313295.php
"The name
comes from the Greek words hippo, meaning horse, and kampo, meaning monster, as its shape resembles that of a seahorse"
So, it's the shape of this organ, which gave its name, unsurprisingly, borrowing from Greek.