Trevenant has an Ability, Forest Mystery.

In game description: Pokémon that have battled this Pokémon become Grass type while they are on the field (but they stop being Grass type if they leave the field). Your Pokémon can move over this Pokémon when using an MP Move.

Practical explanation: A combination of Torterra's Corner stone ability, as well as when an opponent battles Trevenant, they will turn Grass Type instead of whatever type they were. Energy effects will be lost/gained depending on this type change, as well as allowing Trevenant to move them with Manipulative.
Type Name Chance Power Description
Purple Manipulative 32 ☆☆ One other Grass-type Pokémon on the field (that is not frozen) may be moved within its MP range. The moved Pokémon gains Wait. If the Moved Pokémon is one of the opponents' Pokémon, that Pokémon falls asleep.
White Destiny Bond 20 0 If this Pokémon is knocked out, so is the battle opponent.
Red Miss 4 *This section reduced by leveling
White Smash 40 110