M is for Metamorpho

Metamorpho appeared in the "Justice League" animated series episode titled "Metamorphosis". Metamorpho, real name Rex Mason, was an old Marine Corps friend of John Stewart and was transformed by the machinations of Simon Stagg.

Rex Mason worked for Simon Stagg until he fell in love with Simon's only daughter, Sapphire. When Rex discovered that chemicals were being illegally transported by Simon Stagg's company, he quit the company and revealed his affair with Saphire to Simon.

Simon Stagg ordered Rex to be kidnapped and used as a test subject for his new chemical. As a result of the experiment, Rex's body can become nearly any element or chemical he wants. Reminds me a little of Clayface, enemy of Batman.

Metamorpho's creator, Bob Haney, had seen success with DC Comics in 1963 with the titles "Metal Men" and "Doom Patrol", featuring bands of superheroes exhibiting fantastic powers. Haney was asked to capitalize on these titles' popularity with a similar character. Metamorpho, the Element Man, debuted in The Brave and the Bold #57 (January 1965).

Marvel comics' equivalent would be the Absorbing Man (Carl "Crusher" Creel), who has the ability to duplicate the properties of anything he touches - gas, liquid, solid, or even energy sources. This transformation also extends to the clothing and ball and chain that Creel was wearing when the potion took effect.