ToME WikiKhymChanur: Support monsters would be monsters that didn't directly harm the player, but help other monsters of their group to harm the player. The player would want to try to take out the support monsters first to make dealing with the rest of the monsters easier.
Types of support could include:
- Healing the other monsters.
- Boosting the other monster with temporary effects, like speed increase.
- Acting as a mana battery. The support monster would have a very large mana capacity and/or mana regeneration rate, and would cast a low/zero cost spell to transfer some of that mana to other monsters who were running out of mana.
- Not quite "support", but: a necromancer monster that would wait for its comrades to die and drop corpses, then immedietly reanimate the corpses into zombies.
DarkGod: Indeed 
I implemented healers for Bone to be Wild, it's nasty
OldSpider Definitely a great idea. It works many aggravating wonders in games like Avernum when that smarmy healer is camped out in the back behind a sea of meatshields.