Roach

Roach was a member of the Joker's gang. He is found under an abandoned boat near Falcone's Shipping Yard in Batman: Arkham City.

Arkham City Incident
Sometime before the events of Arkham City, the then believed dead Scarecrow managed to capture Roach to use as a test subject for his new fear toxin. When Batman found him, he appeared dead, only to suddenly shout and then die on the chair he was on.

Trivia

 * While the thug isn't named in Batman: Arkham City, he is identified as Roach in a Arkham Knight City Story.
 * Finding the thug may cause a glitch where other Joker thugs have scarecrow markings on them.