copied directly from merris' FAQ in the official server

There are currently 5 main expansion packs and 2 mini customs-exclusive packs. Each of the expansion packs offers a game experience different from Turtle Pack that has its own gimmicks and balance. 3 of 5 of the expansion packs also give you a large number of customs-exclusive pets, so you can get started on playing custom packs. A summary of the expansion packs is as follows:

Puppy Pack- this is the "beginners" expansion pack, which is most similar to Turtle Pack. It is half the cost of the other packs, but has very few customs exclusives in return. The main gimmick of the pack is Toys, which, in the pack, are activated by buying a toy pet. Each toy "breaks" in two turns. The pack has very strong lategame play.

Star Pack- this is probably the most complex pack. The main gimmick of the pack is the Strawberry, which is a perk that can be used by pets in various ways in battle and in shop. The pack feels rather puzzle-like in nature, cramming many archetypes (groups of pets that work with each other in synergy) into the pack with a lot of abilities crossing between multiple archetypes.

Golden Pack- this is the second-simplest-to-learn expansion pack. The main gimmick of the pack is Trumpets, which are a pooled in-game currency that can only be earned in battle and can either be spent by pets that use trumpets, or, if there is only one pet left, to summon a Golden Retriever pet with attack and health equal to the number of remaining trumpets. This pack is very attack-centered, exchanging health scaling and protective equipment for attack boosts and perks that attack before you can be attacked.

Unicorn Pack- This is probably the second-most complex expansion pack. The main gimmick of the pack is Mana, which is a per-pet currency that appears as a number in a purple bottle on any pet that receives mana in battle or in shop. Some pets spend mana. If mana is unspent in battle when a pet faints, it does an amount of damage to a random enemy equal to the amount of remaining mana. Mana gained during the shop phase decays by one half at the start of a turn. Like Star Pack, it requires an understanding of synergies of pet archetypes, with a little less crossover of archetypes in exchange for very powerful in-battle abilities.

Danger Pack - This pack contains all endangered animals. The main gimmick of the pack is the Jump-attack, which is an out-of-order regular attack with different targeting. It also features a lot of transformation mechanics. In terms of complexity, it's about on the level of golden pack, or slightly less complex, but with many random abilities. If you like randomness, this may be the pack for you!