Mimicry now only changes the Pokemon's primary type. By default, G. Stunfisk is Ground/Steel. In Grassy Terrain, its type would change to Grass/Steel. It would be Fairy/Steel in Misty Terrain, Electric/Steel in Electric Terrain, and finally Psychic/Steel in Psychic Terrain.
Snap Trap is now a Ground type clone of Spirit Shackle/Anchor Shot (80 BP, traps foes with no DoT effect) and also changes type in accordance with the Terrain.
It can also learn Iron Head via level up or TR, and each of the Terrain Moves via TR or Move Reminder.

Was that so fucking hard, Gamefreak?