it's not mythological ...
The ONLY history it has as a name is the clown in It, and Pennywise the Dancing Clown is not even It's real name or identity, only the form It takes when appearing to humans.
So no, it could not be used as a name for a different character; it would be like creating a character and insisting his name is Harry Potter or Winnie the Pooh; it would be nonsense.
As a pet name, well, fine, but why?
You're entitled to your own opinions. You're not entitled to your own facts. -- Daniel Patrick Moynihan