Arena Trap

Prevents opponents from fleeing or switching out. Eluded by Flying-types and Pokémon in the air.

Full effect

While this Pokémon is in battle, opposing Pokémon cannot flee or switch out. Flying-type Pokémon and Pokémon in the air, e.g. due to Levitate or Magnet Rise, are unaffected. Pokémon with Run Away can still flee. Pokémon can still switch out with the use of a move or item. Overworld: If the lead Pokémon has this ability, the wild encounter rate is doubled.

In game description

Prevents opposing Pokémon from fleeing from battle.

Pokemon with Arena Trap

Using Arena Trap in Cobblemon

Abilities are passive and always active, so Arena Trap 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.