Here on Fairy Tail Ethereal, there different types of pets. Support, Combat, Exceed, and Companions.
Everyone gets to start with one free D-rank pet.
Support pets are pets that support your character outside of combat, they have no combative abilities and cannot die nor participate in battle. They have passive abilities such as being able to speak the human language.
Combat pets are pets that have spells to aid their owner in battle. Combat pets can participate in battle and fight alongside their owner. This also means they can die in battle.
Exceeds are known to be the companions of slayers. They have the ability to talk, fly, as well as grow ten times their normal size. Exceeds can carry up to three times their own weight. Exceeds are similar to combat pets, they can participate in battle but that also means they can die in battle.(only slayers may have exceeds)
Companion pets, are pets that are just there for the person as a companion. They have no passive abilities or spells. They cannot participate in battle nor can they die in battle.
Combat Spell Slot Rules and Restrictions
D-Rank: 1 spells MAX
C-Rank: 3 spells MAX
B-Rank: 5 spells MAX
A-Rank: 7 spells MAX
S-Rank: 9 spells MAX
Support Pet Passive Rules and Restrictions
D-Rank: 1 passive MAX
C-Rank: 3 passives MAX
B-Rank: 5 passives MAX
A-Rank: 7 passives MAX
S-Rank: 9 passives MAX
Now, as far as Support, Combat, and Exceeds go, if you're a low rank mage and you were to buy a pet or companion that was much higher rank than you, it would not listen to your commands. So, there is a limit as to at which ranks you can buy certain higher rank pets.
D-Rank - Up to C rank pets, companions, and exceeds.
C-Rank - Up to B rank pets, companions, and exceeds.
B-Rank - Up to A rank pets, companions, and exceeds.
A-Rank - Up to S rank pets, companions, and exceeds.
S-Rank - Up to S rank pets, companions, and exceeds.
SS-Rank - Up to S rank pets, companions, and exceeds.
X-Rank - Up to S rank pets, companions, and exceeds.
For combat pet spells, please follow the Magic System Rules