Hunter’s pets haven't changed a whole lot since Classic World of Warcraft, but they have become stronger by scaling with the owner, so the stronger a player gets, the pet will gain additional benefits as well.
Hunter’s pets also received some new abilities, notably avoidance, which will reduce passive AOE damage they take while in combat. If there was one annoying thing about hunter pets in classic, was the random deaths during a boos fight, resulting in a considerable DPS loss.
Pets will still need to be leveled from the level they are caught at and have limited training points based on their level and loyalty.
There are three areas that will be the focus on what pet is good for what situation.
The best pure damage pet is a Ravager. Ravagers can learn both the Bite and Gore ability and have a high damage modifier of 1.10.
One of the new Marksman talents added in TBC “Go for the Throat” gives your pet 50 focus whenever you crit. This talent can be picked up as early as level 62 if you are playing the basic BM spec going into TBC.
The best Pet for Arena will be a Scorpion. These beasts can train an ability called “Scorpid Poison” that will apply a debuff on the enemy, that stacks up to 5 times.
If this is applied after a hunter applies Viper Sting, the enemy team will be unable to dispel the Viper Sting from themselves or their team.
Scorpions can be used in Battlegrounds also, but it is not as important as having one in an Arena match.
There are many pets that can be used for leveling, and some people will argue that if you are specialized in Beast Mastery talents all pets will excel in leveling content.
They can both pick up growl like most pets for the single taunt and can learn Screech an ability that affects all those around them causing high amounts of threat. They can also learn Swoop, a variation of the dash ability which gives them move speed, which helps to move from mob to mob.
There are still only 3 stable slots for hunter pets (1 active 2 stored in thestables) so choose wisely.