Imposter
Transforms upon entering battle.
Full effect
This Pokémon transforms into a random opponent upon entering battle. This effect is identical to the move Transform.
In game description
The Pokémon transforms itself into the Pokémon it's facing.
Pokemon with Imposter
Using Imposter in Cobblemon
Abilities are passive and always active, so Imposter costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.
See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.