There are 7 variable stats for pets:
Health: The number of hit points the pet has. Normal players have 20. Therefore every 2 health points= 1 heart
Armor: This value is subtracted from all incoming damage. Maximum of 10 armor.
Strength: This is the damage the pet does with each hit. Again, every 2 points = 1 heart.
Range: This is the detection range for the pet, how far away mobs can be detected to attack. Maximum of 15 range.
Speed: This is the speed at which a pet will move while pursuing a target.
Attackrate: This is the number of seconds between projectile shots and has no effect if the pet uses melee. The minimum internal rate is 0.5 (twice a second).
Healrate: This is the number of seconds between heals of 1 point. (Half a heart)