Concept drawing of Caliban by John Byrne from 1978.
John Byrne from X-Men Companion II on the creation of Caliban
He was going to be an incredibly ugly Living Cerebro kind of character, whom we would never see entirely. We would only see bits and pieces of him. And he would live in this dark room in the back of the mansion and Kitty would relate to him for some reason. Chris defined the situation as being she can’t stand Nightcrawler because he is “wrong” somehow, he is a human done sideways, but Caliban was so hideous and so deformed that he was a broken thing, like a bird with a broken wing, and she could relate to him on that level. And it was going to be a very trying thing for Nightcrawler, the fact Kitty could relate to Caliban and not him.
On why the character never made it into the book.
Because Shooter killed the concept of a second team of X-Men, the X-Men in training, because he said it sounded too much like the Legion of Substitute X-Men, so it died right there and Kitty became the only one who ever made it into the book.