To truly know a creature, slither a mile in its tentacles.

Rise of the Eldrazi

Renegade Doppelganger

Whenever another creature enters the battlefield under your control, you may have Renegade Doppelganger become a copy of that creature until end of turn. (If it does, it loses this ability for the rest of the turn.)

Whenever another creature you control enters, you may have Renegade Doppelganger become a copy of that creature until end of turn. (If it does, it loses this ability for the rest of the turn.)