Welsh has a lot of first names: we were a few centuries behind the rest of Europe is adopting last names to identify people, and even then for a while it would be still be the "son of/daughter of" construction. Like, I would be Ifan ap Martyn, for example.

Ifan is a Welsh variation of John, and Ianto is a pet name for Ifan. It's basically like calling me Johnny instead of John. Exactly how that came to be I really don't know, but I don't know how people got Hank from Henry, either.

Nicknames aren't always about shortening a name; in old English they were a way to express relationships.