I regret changing my Experiment into a different species. Tried getting one by zapping for some time. Gave up and used a Miracle Potion II to get it, only to change again a few months later. ^^;
I regret using two Blacklight Potions on blacklight cybills that I eventually ended up adopting out or changing into something else. :') I LOVE the blacklight cybill but I just have not been able to stick w/ a character idea, it's tragic
I have a lot of regrets that involve wasting sP on changing pets around but my blacklight cybill problems stick out, haha :P

[flower=lea]
I regret making so many fan-pets.
They're easy to make, but I've started to outgrow some of the shows, so I don't really care a whole lot about them anymore. :S And I spent so much on them!
This! This has been my problem more than once! I get so excited that I don't think my ideas through in a long-term way. There are a few things I regret a bit besides that - a name or two that I gave up. I once adopted a pet from the pound that had a great name, but put her back quickly for a name I thought I wanted more. Well, that pet didn't pan out and I ended up wishing I still had the first pet. Serves me right I suppose. One of the things that bothers me about a few of my pets is that they have a period in their name. Some names I wanted badly enough that I'd rather have it than not, but I check up on the names every once in a while in case the non-period version has become available. Of course, then I'd have to pay a ton to change the name of my pet, or else I'd have to adopt them out and create anew (which I kinda hate doing as I've become unhealthily attached to them). One thing I'm really happy about is still having two of the very first pets I ever made. Kamots was my first pet ever, and he's still my baby. Chemukh came shortly there after. :3
Before angelic potions went cash shop only and were thus a lot cheaper, Verdwijn was an Angelic Kumos. I re-potioned her (I forget what to, I think Glacier) and then, wellp. (I also regret abandoning a huge number of character concepts, and thus wasting a lot of sP on elixirs and potions I'd just re-do later, but there you go.)
Though something I don't regret: I still have all five of my original pets, and all five of the additional ones from my first GA, though I've changed about half of their names (Omicidio, Heilig, Verdwijn, Geerdet, Mattock, Bohemia, Biala, the pet who became Eight Melodies, the pet who became Third Impact, and the pet who became PK Love). I regret some of their original names (they were misspelled or in-jokes with friends I'm no longer speaking with), and they've completely changed character, concept, and pet-colour combos, but the fact that their IDs are still the same as ever means something to me, idk.
I don't have too many. Mainly pounding pets I really want back now. Especially my first pet, Kymei. I also created a lot of nice names in 2008 who I later pounded when I decided I only wanted one pet. lol I couldn't even tell you their names anymore. I don't remember at all. I'm just glad that Midnight was the one who I kept and now I'm completely happy with my pets. I just wish I had more slots now. haha
Edit: spelling

I kind of regret pounding the rw pet I had to make space... I did it to make the tribute pet for my cat, so I don't regret it that much... but I do sort of miss him.... and his color. He was a Galactic Harvester, and I think they're so neat, but he just didn't evoke emotions in me.
And I soooort of regret making my pets Arien and Gwyn, partly because I don't like the "Passerine" parts of their names (Arien and Gwyn were, of course, taken), and partly because I really kind of don't like having the admission out there that they're from my novel. I'm a pretty private writer, so it makes me feel... idk, self-conscious or something.
I sorta regret making Ojo a Nostalgic Torrent. I love the pet color, but it does't work for that pet. I've been too stubborn to spend the money again hoping for a pet name switching ability in the future.
I still wish I had Ywaar, but I'm glad a friend has him at least.