Do you get anything for releasing pets?

No, you don't get anything from releasing pets.

People release pets for space, to save gold on feed and/or to preserve the value of projects so that people can't buy and produce a similar project with a lot less effort thereby devaluing the original project.

Edit: Thought it worth mentioning for completeness that the Spring Festival had a contest where the top x number of people to release a certain species of sylesti won a tagged of that species.