Hippos can be found around the Killmaresh coast.
There is some locations where you can found them, the first one is IMO the safest spot to complete the achievement in the inside the city as the picture below shows:.
And these are all the points that I know so far, of course there are more points that I am missing in the image. Specifically located around the coast of the Central Steppe, the Southern Steppe and the Green belt. However, the places I have marked on the map are the safest and easiest to find.