I suppose war creatures make a better defense then empty space, but thats about it. Even grizzlies are basically only good as ambush catchers, bait, and for fighting off other creatures. Plus you have to be careful with bears, they can drink your alcohol, although trained ones at least seem to know better.
Thats the thing though, if you only send a couple war animals of any kind they will be slaughtered.
Don't send only a couple of them. Send them all!
Each species has its own local population cap, so you can have populations of dogs, grizzly bears, black bears, polar bears, crocodiles, alligators, cave crocodiles, tigers, giant tigers, lions, giant lions, wolves, cougars, etc.
If you create your entrance hall in a clever way while also allowing those in your meeting zone to see any enemies, the animals will all attack them and pile onto the attackers. Yes, some will die, but they should quickly reproduce to replace any losses. The idea is that while the goblin is busy killing one dog, there are 10 others biting him. The goblin then bleeds to death nearly instantly from all of those bites.
If you don't want to rely on animals they're still very useful as arrow magnets. Each archer/crossbowman only has a small amount of ammo on him. If you make sure all attackers pass by a pit full of animals, so that the attackers can see into the pit and can get a clear shot off, they will empty their quivers at the pile of animals. Make sure they're firing at max range and you will greatly reduce the losses taken. Any losses are also quickly replaced due to breeding. By the time the invaders get past the animal pit they will be out of ammo, and will then use their crossbow as ineffective clubs.