The only real way I've managed to effectively use animals in combat is to get a huge herd of something and use it to swarm the enemy. ex: Setting the allowed area for a boomalope herd to be the area that an enemy siege is being set up in. Trying to use them as auxiliary as intended doesn't really work as they just get mowed down by enemy forces.
One thing that would help would be to introduce barding for animals so that they can be tankier.
Another potential solution, though far less trivial, would be to allow colonists to use certain animals as mounts. With mounts you can expand combat in a lot of ways. Allow a mount and its rider to both be in melee against the same target, have mounted colonists more likely to hit in melee and less likely to get hit (w/ mount getting hit instead), allow colonists to fire weapons while their mount is moving (albiet at a reduced accuracy). Heck, even just being able to use a mount to get into a flanking position more quickly would be useful.
One thing that would help would be to introduce barding for animals so that they can be tankier.
Another potential solution, though far less trivial, would be to allow colonists to use certain animals as mounts. With mounts you can expand combat in a lot of ways. Allow a mount and its rider to both be in melee against the same target, have mounted colonists more likely to hit in melee and less likely to get hit (w/ mount getting hit instead), allow colonists to fire weapons while their mount is moving (albiet at a reduced accuracy). Heck, even just being able to use a mount to get into a flanking position more quickly would be useful.