Species (except special lines like the Malks and Nicks I have all together, no matter the species. ^_^" ), and I have a baby group for when they are growing. If I get one of an outcome, I move it to it's respective group & if I get extra of the same outcome it stays so I know I won't accidentally trade a split outcome.
Extras are just sorted by rarity: Commom (all brackets), Uncommon (all brackets), Rare (all brackets - even an omg if I had one lol would go in that group xD)
And I do have a small collection group that's comprised of gifts, pound pets, or random pets I just like lol. I coulda adopted it, or sent a bunch of trades for the same pet and more than one was accepted.
There's also the white cats in that group, I collect all white, bi-eyed cats to honor my Peppermint who passed away this year. I had her for 17 years. </3
Oh and I have a free group, it's under commons doubles that I don't really care about. xD