The Mongrel is the first summon that the Witch Doctor gets.
This summon was first seen in the Gameplay Trailer where fans were told that it could be enhanced with the Witch Doctors offensive spells to do additional damage. The Player then demonstrates this by blasting the Mongrel with a Locust Swarm. This buff makes the Mongrel's appearance change to be covered in a green goo as well as cause additional damage with it's regular attack. Later in the same video the female witch doctor is seen hitting her mongrel with a Skull of Flame (although it was called "Fire Bomb" at the time of the trailer) causing it to look fiery and, presumably, do fire damage.
There is also a Sacrifice spell in the Witch Doctor's arsenal with causes the Mongrel to explode and cause damage to all nearby enemies.
As many as 4 Mongrels are seen in screen shots and in the gameplay video for a single Witch Woctor, although it is not known what the limits are, in fact most of the mechanics are not known for sure.