So is there any benefit from releasing big numbers of sylestis? Someone just bought a huge bunch of my hatchlings and then released all of them. Why, is there anything you can gain from it, apart from throwing away your gold to whomever is selling the sylestis?

During festivals there is an event where people relass pets to win tagged pets or special item.

Seriously? Is this an official thing?

Yup, it's a part of the letherbord contests

It should be noted that the Release board is only part of the Leaderboards during the Spring Fest. So I'm not sure why they'd do it for that.
Unless they just didn't notice the decided lack of release board in the Leaderboards.

Only other reason we can think is it was a price-altering spree for a species, though?

Another thought is for the nurture contests. Perhaps someone bought them, nurtured, and released before other players could, boosting their nurture numbers for them alone.

Otherwise, I've had people buy my low priced pets just to release them in a manner of controlling the economics or to lessen the low-vis pets in the world. It's a controversial thing but ultimately, it helps. Think of it like in nature when their are an over-abundance of deer who end up starving because there are too many mouths to feed.

I never minded; they were pets I would have released but it was more time-saving to put them up for cheap, generally.