A "list pet" is a leftover term that meant the rarest pets on CS. Old store pets or high demand pets might be called "lists". If you've ever heard terms like "mains, septembers, advents, octobers" they're all from the same list (that ButterBirdCamo linked).
The list was disbanded sometime in 2018-2019 and since rarities were updated in 2023, users pretty much jumped over to using "
Horror's Guide." A LOT of the "list pets" are now on
tiers 1-6 on Horror's guide. You've also seen "MA" thrown around which stands for "main advent" which came from the old list. It's most commonly a unit of measurement for valuing rare pets:
1 "MA" = 10 old rares | 0.5 "MA" = 5 old rares
An "old rare" is a basic rare from 2010-2011. It was something the community voted on in 2023. A lot of 2008-2009 pets turned into VRs so the baseline had to change.
A quick way to know if your pet is a list pet is if it has an extremely rare or OMG so rare rarity tag vvv
Extremely rares are pretty much guaranteed to be at least 0.5 MA and OMGSRs are guaranteed to be 1 MA. And URs and store pets of course tend to be much, much more. Some pets are worth 10s of MAs.
There are exceptions of course and really every pet is different. There are even rares and very rares that are worth much more than their rarity tag. If you are wondering about a pet, definitely feel free to PM me and I can help value it. I understand the guide can be long and intimidating and frankly there are users who don't have the time to want to understand it ^^ and that's A-Okay!
Hopefully this was comprehensive enough to answer your question fully ^^