I read a lot about that LobsterBoy guy (who was a real asshole, if his children and ex-wife are to be believed). I think they actually move, but they only have two really fat fingers. He was supposed to be a real Ladies Man, if you get my drift.
What do you mean “developed?” The exmples I’ve seen, like Bree Walker were the result of a genetic defect so they were born that way. She passed the deformity to at least one of her children. As for ability to use defused hands they probably have to learn muscle coordination as does any baby learning to use normal hands.
The “lobster” condition is missing or fused digits in the hands and the feet and it called ectrodactyly. I did a few quick searches but couldn’t find any images to show the difference.
I lived in San Diego in the early eighties and never realized Bree’s condition until I read a magazine article a few years later. She often had her hands on the desk but few people noticed.