Hattrem cannot be found in the wild.
After looking online and seeing many other people not being to find it, and not being able to find one myself with an Encounter Power 3 boost for Psychic types (did find a random shiny Psyduck though), this is the conclusion I've come to. Either it wasn't supposed to appear at all, or something in the programming is causing it not to spawn, but whatever the case, it just does not appear in the wild. Of course, there are still ways to obtain one, those being:
Evolving a Hatenna
Hatenna spawns in all the places the Pokedex lists, so find one near a pond, catch it, and level it up to level 32. I find it easiest near the ponds in South Province Area 4, but the ones near the pond West of Medali spawn Hatenna around level 30. Those are a rather uncommon spawn, but with a Psychic Encounter sandwich, you should find one soon (look for Hatenna on the side with the 2 trees because they don't tend to spawn on the side closest to the wall).
Find one in a Tera Raid
Hatenna, Hattrem, and Hatterene can all be found in Tera Raids, so if you're lucky, you should find one.
So yeah, evolve a Hatenna or find one in a Tera Raid. Sorry if this wasn't the answer you were looking for, but hope I helped regardless.
