what pets can you choose? is it every pet available to the site? is it a set pool? is it only the defaults? how many choices do you have? this is a very expensive potion for my limited budget so i have to be sure it's going to be worth going bankrupt for xD

more specifically, i want to somehow change my main pet into a Zentu if possible. i do not want to make a new pet as i've put a lot of time into this one already and don't much want to start over with someone new ♪

It's any existing pet species, including experiments :)

However, if you're looking to get a Reborn Zentu, since I think you mentioned changing a Reborn pet, it's cheaper to get a Zentu elixir and a Reborn Potion (250k from Five Seven Five + ~7.5m in user shops versus 53m for the Miracle Potion II, which is slightly cheaper than the current going price of the CSC)

THERE'S A ZENTU ELIXIRRRR??? i didn't know you could change your species via an elixir, i thought it was just colooorrrsss~~ ugh-- this is why i ask the pros!!! xD

Yep! Elixirs change a pet's species, but they generally only come in the "basic" colors that are in the same pose as the Common color of that pet (there are a handful of exceptions like some Bloodred elixirs, but I don't think those exist for all pets) c: