This fierce-looking Punchbag just happens to be Bob's brother, a rough-and-tumble guy who isn't there to just take a beating. Having a difficulty of 100, he's ready to deal out some damage to your pet, dare you take him on.
Sid can be battled when he occasionally takes the place of Bob in the Battledome, and if you lose to him then you'll get the "Punchbag" avatar.
Sid had a brief cameo in the Curse of Maraqua plot, being ripped to shreds by one of Swordsmaster Talek's blades.