Is there any advantage to releasing pets besides clearing up space in the stable/hatchery? I've seen someone buying the pets I sell for cheap then releasing them but when I experimented with releasing one I didn't get any money or anything as far as I know so I was really confused why that person was buying and releasing pets.

If the pets are over 42 days old, then releasing them increases the chances of finding wild sylesti eggs in the Lost Grove (current endgame area for level 60+). However, some people go through the sales and buy up cheap pets and release them to help the economy stay strong. Pets sold for low amounts of gold are usually seen as pets no one really wants, and therefore likely only going to sit in sales and clog things up.

But, yes, the main purpose of releasing pets is to clear up space in the stables, when you either can't, or don't want to, sell those pets to other people.

Releasing pets over 42 days of age increases the chance of a nest spawning in the Lost Grove (level 60+ area). A player buying cheap pets and releasing them is trying to clear the sales markets most likely, that or maybe they've forgotten about 42 days of age part when it comes to the nest spawn.

Oh gotcha, thanks!

there is more then one adventages actualy :

1. it remove cheep, often dishwacher, unwanted pets from the marquet so they won't stay there eternaly
2. it actualy increass the values of the speacie as it clean the pets that are not valuable
3. if the pet is over 42 days it boost your eggs encounter in the lost grove
4. during festivals it can be use with the leaderbord or win special tagged pets