Sorry if you've figured it out by now but I'll try to explain best I can.
When people refer to 'the list' they're referring to a rarity list that was put together by the community to see which pets are the most rare on the site. This list is often used in trades to make sure valuable pets are traded for equally valuable pets. Rarities can be seen when clicking on a pet and looking at the colourful rarity bar in their profile.
Considering there's a lot of pets with high rarities, the list is there to provide more info. Older pets are often more valuable than newer pets, even if they're the same rarity.
The list starts here. It begins with OMG so rare pets and continues through to Very Rare pets.
For example: when someone says they're trading for High Mains only, it means they're trading for certain pets on the list I linked above.
Not everyone follows the list but it's a useful tool to gauge rarities if you plan on trading lots. :)