Elasticity – The ability to stretch, deform, expand or contract one’s body into any form imaginable.
Elastic Lad (DC) is a superhero with this power (DC Who’s Who V1 #7).
Elongated Man (DC) is a Justice League member with this power (DC Who’s Who V1 #7).
Mister Fantastic (Marvel) still has a more or less human body that can become more elastic.
Plastic Man (DC) Plastic Man has the additional power of substance mimicry. Plastic man’s body is made out of some hard to define substance that is like plastic. This substance is far more durable than human body (DC Who’s Who V1 #18).
Janus Stark is British hero that has abnormally flexible bones but probably does not have elasticity as a superpower. Some of his seventies adventures were reprinted in Albion Origins (Titan).
Plastic Sam is a parody of Plastic Man in MAD #14.
Ronald the Rubber Boy – Archie’s Madhouse #52 (1967)
Rubberman – Albion #4
Shape (Marvel) – Squadron Supreme V1 #11
Tack – Ben 10
The Balls Brothers – Heroes for Hire – 2000 AD #1130
“The Old Hag” – Adventures into Terror #10 (Atlas)