You cant. You can only make hippodown pregnat.
hippotas is on iron island
in the Maniac Tunnel can you find them
In the elite 4
They are in the maniac tunnel.
A trainer on iron island has hippotas
route 214 it may be rare catch
in vellstone city
Have you seen a trainer with one? if so go to your pokedex search for the hippotas and go to "Area"
You find hippopotas (note I spelled it right) in the maniac dude's cave
evloves at level 34
you see it in vellstone city
IT Evolvez in LV.34