Attack dogs would be very cool. They are extensively used in the world today, and it would make sense if they were in the Tremulous universe as well. Although PETA might have succeeded in removing dogs from the military by the time we get there…
Gameplay effects would be hard to foresee. They are (naturally) autonomous and melee-range, and probably faster than humans. I don’t know what their bite would be like against a dretch’s exoskeleton (it does have an exoskeleton, right?) but it sounds like it would make a good dretch-killer, at least as long as its enemy is on the floor. Anti-structure capabilities would also be hard to work out.
Suppose a dretch goes head-to-head with a dog. Naturally, the dretch being played by a human and the dog by an AI, we should reasonably expect the human to win. (Note that this doesn’t apply to turrets, with which encounters can be controlled by the player as the turret is immobile.) That means that the dog would have to have a weak enough bite not to one-shot the dretch. This however creates a problem: dretches are a good enough alien, especially in swarms, but they still aren’t powerful alone (disclaimer: except with very skilled players). If the dog is going to be weaker than the dretch, and AI-controlled, it isn’t going to be strong at all, especially against large aliens like goons, and likely to be regarded as a waste of credits.
I have a counter-proposal to Sunphantom’s idea: tactical drones. Drones with specific weapon attachments (e.g., rifle drone, shotgun drone, laser drone, maybe chainsaw drone) would be purchasable at an armory. They could come with a basic remote control that toggles between two modes: “guard this point” and “follow me around.” Perhaps they automatically return to the nearest armory when their ammunition is depleted. Drones are (in)famously used by the US military today, and we could reasonably have intelligent robotic drones when we have robotic turrets that can identify aliens, and pocket-size nuclear reactors.
In my opinion, drones are better than dogs in several gameplay-related ways. First, because they use ranged weapons, they keep the human team focused on long-distance attack. Because they are robots, they aren’t necessarily either quick movers or well armored, making it easier for individual dretches to catch them. They would still have an effect of “crowd control” when with its controller, as some of the dretches go for the human and thereby make it easier for the drone to pick them off. Drones would also be capable of boosting the defense of the human base without additional BP; while the new turrets make this somewhat overkill, drones would definitely increase humans’ tactical options (like guarding a “control point”, e.g. intersection, to reduce surprise attacks from the rear).
TL;DR dogs are a cool idea, but would have to be nerfed considerably so as not to overpower dretches; drones keep the human focus on long range and provide a variety of tactial options.