Mankind's (Mick Foley's) sockpuppet.
Mr. Socko is a dirty gym sock with eyes. It's usually kept in the pants of Mankind. Usually makes an appearance in order to help apply the mandible claw, Mankind's finishing move.
I believe Mr. Socko was first introduced when Mankind was visiting the evil Vince McMahon in the hospital. Mr. Socko was a key component of the Rock and Sock Connection.
As Mick Foley has retired from active wrestling, one assumes that Mr. Socko has also retired. He will be missed.