Biological Manipulation (freak) – The ability to turn another being or yourself into a freak/monster.
Mister Hyde (Marvel) is a scientist that can turn into a monster (Official Handbook of the Marvel Universe V1 #7).
Monster Girl (Image) can turn into a monster and back at will (The Official Handbook of the Invincible Universe #2).
Turning a hero into a freak is so much fun, BWAHAHA (Action Comics V1 #284, Aquaman V1 #21, Batman V1 #162, Brave and Bold V1 #68, Challengers of the Unknown V1 #50, Jimmy Olsen V1 #53, Jimmy Olsen V1 #59, Lois Lane V1 #66, Rip Hunter Time Master V1 #28).
The Falange is a former officer of the Spanish army that has been transformed into a monster by an experimental combat gas and is part of The Chimera Brigade (Titan).
Vurk’s normal form is that of an alien monster in the Ultraverse (1993 SkyBox Ultraverse).
The American Kaiju (Marvel) is Godzilla in the service of the US government (Avengers Standoff).
Cypher (Marvel) – New Mutants Forever #5
Ghoul – Field Guide to Cthulhu Monsters
Meenlock – TSR 2158 Monstrous Compendium Annual Volume 2
In Journey into Mystery V1 #79, a man decides to become “The Midnight Monster” but comes to an ironic end.