Combining Mob Shells/Skins into one Mob

Hey guys,

I'm completely new here so I apologise if this is a really nooby question. I was wondering if it was possible to combine 2 mob skins/shells/whatever you call it into 1 mob? I was hoping to combine the spiders legs of to the back of a person kind of mob to make it look like he had tendrils/tentacles coming out of its back, kind of like Doc Oc from spider man, but cooler haha

Cheers guys