Anyway... I thought about it a bit more too.
In my mind's eye, a guard dog to me would seem to be a big dog like a german shephard. The big dogs seem to be covered under the "Riding Dog" entry of the MM.
However, since the PHB only charges 25 gp for a guard dog, I would lean towards using the regular "Dog" entry and giving the canine some known tricks. These tricks would be:
Attack
Come
Defend
Down
Guard
Heel
The dog would obey it's master, though a charm animal spell to turn the creature on it's owner.
I agree with Philosopher that adding animals to the arena adds a different level of play... but if they are available to everyone to purchase, then the playing field should still be level, right? (Everyone is going to get a guard dog now
) As players we have choices to make with regards to how we will spend our resources, right? ("Do I get two vials of acid or one flask of alchemist fire?" "Do I get a shield and pick up power attack, or do I wield two weapons and get Two-Weapon Fighting?")
My understanding of this game is that its pretty much hack n' slash. You get 10 rounds to kill off as many opponents as possible. So the idea is to either try out character classes and see how they'll stand up in combat, or look to build the most powerful character possible and try to dominate!
ON A DIFFERENT NOTE: Devalero, you haven't spoken much about the environment we'll be fighting in. Will each fight be in the same location? Are there natural hazards within the arena? Will there be natural cover or concealment in the arena? Will all the areas in the arena be lit? Will there be NPC creatures also looking to do battle with us?
Just curious?
Also, do characters have a round or two prior to the fight match beginning to cast buff spells on themselves? (i.e. Mage Armor)
Thanks!