How to free a horse?

As said by Seiyria, you can kill a horse but that is a bit creepy.

So otherwise, there's a guy on the way to Gerudo who asks you to sell him a horse for 300, so it might be better to give your horse to him rather than killing him ;)

And I haven't tried this yet, but it seems that if you talk to the guy at the stable about registering a new horse when you already have 5, he will ask you if you want to free one.

One possible way to set a horse free is to take it to Hyrule Castle and feed it to a Guardian.