What type of contract would say that the creator can retain IP rights, as long as the company can use and sell the work too?

"retain IP rights" is rather vague. It sounds like you're talking about a nonexclusive copyright license, though.

A licensing agreement. There are hundreds if not thousands of details that can go wrong in such a contract for either party, so make sure you consult with a specialized IP attorney bfore signing one.

A guide to selling/licensing IP. It's so much easier answering questions that I've already written the answer to...

Thank you