The ruminations of an idle mind:
Several English words such as potion, potable, potables, poison, derive from the Latin root potare, poto "to drink". In all cases these words are pronounced with a long 'o' as in 'oh'.
The noble hippopotamus, according to OED:
derives from Late Latin hippopotamus, from Greek hippopotamus "riverhorse," an irregular formation from earlier ho hippos potamios "the horse of the river", from hippos "horse" (see equine) + adjective from potamos "river, rushing water"
In turn, the word element potamos- is described:
word-forming element meaning "river," from comb. form of Greek potamos "river," perhaps literally "rushing water," from PIE *pet- "to rush, to fly" (see petition).
Unfortunately, I've been unable to find a resource that indicates how potamos- is pronounced in Greek and Latin. My reasoning is that, like the root potare, the element is pronounced with a long 'o'. However the OED excerpt above suggests the pronunciation might also be "pet-amos".
This raises two questions:
How is potamos- pronounced in Greek, and does it have any relation to the Latin roots potare and poto?
Where does the "paw" in "hippo-paw-tamus" come from? It seems to me that in order to be consistent with other English words, which preserve the poh sound, the pronunciation ought to be "hippo-poh-tamus" or possibly "hippo-pet-amus", and yet this clearly isn't the case.
I realize this subject is more than a little obscure, but any insights would be appreciated.