A Pokémon with the Beast Boost Ability will have its highest Statistic increased when it knocks out an opposing Pokémon. Specifically, the stats that can be increased include Attack, Defense, Special Attack, Special Defense and Speed.
Beast Boost doesn't have an effect in the Overworld.
Pokémon With Beast Boost
Related ThreadsList of confirmed new Alola Pokémon, Alolan variants & Ultra Beasts! | Updated 14th November 2016 - last post by @ Nov 14, 2016
Legendary Beast Theory - last post by @ Jul 4, 2016