I've often wondered about nick names and how they came into being, especially when the nick name doesn't seem to have much to do with the actual name.
Take Harry. If we're talking about the prince, yes, I'd definitely take him. But his real name is Henry. Both names have the same number of letters, so how on earth did Harry become a nick name for Henry?
If we think right back to King Henry the Eighth, he was also nick-named Harry, So Henry/Harry has been around a long time.
The nick name for John is Jack. Margaret is Peggy, Dorothy is Dolly, and Richard, Dick. I think they, too, are probably Olde English.
Growing up, I had a cousin who, like me, was named Lorraine. To differentiate between us, she was known as Lorrie. I was Truck.