There are no horses either. Neither are there goats, sheep or camels as far as we have seen. As the resident cowboy here, I am suddenly feeling very alone. I figure the main reason we don't see animals is strictly financial. If there were herd animals, TPTB would have to hire not only the animals but the wranglers to handle them. Animal expense would translate into less money for other cool stuff like FX and actor's salaries.
This also raises an interesting point. Most of the cultures we have seen have been largely agricultural or hunter gatherers. I seem to remember that Teyla said that the Athosians regularly moved their hunting villages around to make it harder for the Wraith to find them. It seems strange that there are not more nomadic tribes that move with their herds in a constant search for food. This would also serve to make it somewhat easier to evade the Wraith.
Cattle on the hoof would be relatively easy to drive from planet to planet through the Stargate as well....unless they were raising Texas Longhorns. (The horns might get stuck in the gate.)
