Metamorpho

Metamorpho was the result of Project: Meta, an attempt to turn mud obtained from Clayface into weaponry by Stagg Enterprises

Using a radioactive shard from Egypt (assumed to be the Orb of Ra) to filter cell mutations and reconstruct its cellular structure, the resulting creature began to weaken. To stabilize it, they used organic material (presumably from a briefly mentioned 'lackey', again, assumed to be Rex Mason) to create Metamorpho, a creature that could change its chemical make-up to resemble several base elements. Not yet understanding all of its capabilities, the creature was kept inert in a tank.

Before Arkham Knight Incident
The project's head scientist, upon seeing Batman and Deadshot snooping around his lab, opened fire on the pair with a laser rifle, hitting the tank storing Metamorpho and freeing it.

After a brief fight, which resulted in Metamorpho escaping into Stagg Enterprises' parking lot, Batman was engulfed. Using his Line Launcher, Batman escaped and threw three Sonic Batarangs into its body, which exploded inside it.

Batman then took some samples for later study.