I think the title says it all. Are giant mantises good for war animals? I've got a few in cage traps and I'm hoping that they'll be great for killing elves and gobbos.
Never tried them, to be honest.
But, like a lot of animals, they probably won't hold up well if the enemy is well equipped. Killing elves largely depends on how many archers there are. If the animal can close to Melee, it's got a fighting chance since elves are poorly armored.
Goblins are a different story. Their metal armor makes them very resistant to animals, even big ones. That said, some of the giant animals are still relatively effective, espeically if backed up with archer support.
But there is a larger problem here. Giant Mantis only live for 1 year. Because of that, it takes a lot of micro-management to keep their population going since any one mantis is only ever going to live long enough to produce one clutch of offspring.
You could plop the whole set down in a single room and keep them breeding at will, but the FPS hit you're gonna take will probably not be worth it.
There are only a few vermine giants that are worth keeping around, but most of those only ever show up in groups of 1, so getting a breeding population going is difficult at best. The only one that is relatively easy to get your hands on is giant flying squirrels since they spawn in groups of 3-5, but they only appear in temperate forests.
There is a final problem here. Mantis are not agressive. As such, they will tend to flee from combat, rather than stick around. You could fix this by modding their raws to allow them to be trained as war Mantis.