I do believe you require approximately two fully-equipped and high-morale legions to kill a singular Hippo in that game, no joke. If you see 2 or 3 Hippos in a singular area, you'll need a substantial Army to clear them out.
EDIT: If you're lucky and can push the Hippos out to the river somehow, your Navy can bombard the Hippos with impunity, but it takes a long time for those ships to kill a Hippo. (I've literally built the Pyramids before the Hippos died. So a really, really long time)
The other plan is to set up a lot of Police Stations. Police don't have morale-stat and will fight to the death (in contrast: your armies will break formation and retreat). Each time a Police dies, they hire a new citizen rather quickly, eventually your infinite stream of Policemen kill the Hippo. This works because Hippos only attack citizens, never buildings. (In contrast, if you actually had an approaching army, the Police would die, and then they'd destroy the Police Station... so no new Police would come out to defend.)
Also, immigrants come extremely quickly in Pharaoh. You pretty much convert immigrants into Police (and then subsequently dying) at unrealistically high speeds. Armies actually need "training", "Morale" and all that stuff...
Are Pharaoh's Hippos related to Dwarf Fortress Carp, perchance? Because this situation sounds familiar.
Its the only settler game I've played where I enjoyed the siege warfare at all.
Direct link: https://en.m.wikipedia.org/wiki/Farm_(revenue_leasing)