Looters: Looters are your basic starter baddies. They're never mounted and have little if any ranged ability. Looters in Nova Aetas are significantly better equipped for melee than in native. You'll find that some have Agonic armor with defense in the 30s and steel helmets with defense in the 30's or low 40s.They're easy to take ranged from horseback if you have the patience and want to stay out of reach or just want to increase your ranged skills. They're also good to ransom if you can knock them all out as they're significantly less dangerous than other neutral enemies.
Mountain, Taiga, and Forrest bandits (Taiga found in NE near Vageirs, Forrest mostly central with Swadians, Mountain mostly SW with Rhodoks): Mountain, Taiga, and Forrest bandits are a step up from Looters. (each group varies only a little) While lightly armored and also not usually mounted, they can be initially difficult to fight alone since many of them will be carrying bows in addition to their melee weapons. You can fight them alone if you've got some heavier armor or with some backup. They're relatively easy prey for trained army troops. They'll often give short bows, light armor, and short melee weapons as loot.
Sea Raiders/Pirates (Mostly along north shore of map): Sea raiders or pirates have medium Nordic armor and helmets. They prefer melee and throwing axes though some have bows, short spears, or Nordic swords. They are never mounted and so can be easily out maneuvered. They have a limited number of throwing weapons and so can be circled until they run out. They're pretty tough in melee but if you can manage them they give fairly good Nordic shields, helmets, Nordic swords and medium armor.
Steepe Bandits (Found in SE with Kergits and Sarrinds): Steepe bandits are always mounted on light steepe horses. Many are horse archers, have light melee weapons and light armor. They can be difficult to catch in battle but I've found they usually get cleaned out by heavier cavalry. Fairly weak if unhorsed. They will often give horses, nomad or Kergit bows, and those nice Kergit arrows as loot. Good source of horses for pack animals or companions. They can be fairly dangerous toward game start and since they're fast on the world map they can be difficult to avoid.
Cathar Extremists: Cathar Extremists are radical holy knights who attack anyone who aren't Catholic Christian. They are situated in Swadia and are often found patrolling near near the forests around Ibaeran. Their hideout is called a Cathar Castle which usually appears in these forests. They are always mounted on black warhorses, have heavy armor/helmets, and carry greatswords. Cathars are fairly dangerous toward game start but if youi can take them out give good loot. A strategy I've found works well is to take them in a circle with a lance and as the ones in the back of their line turn to cut you off, take them or their horse out with a couched lance. Once they're unhorsed they're significantly less dangerous. The Cathar Achilles heel is their lack of ranged ability. If you are or have horse archers they can take them out with relatively few losses.
Deserters (varies by location): Deserters are made up of the army troops of they area they originate from. They often don't wander too far afield from their nation of origin but it's not unheard of. Deserters are often all of one kind of troop such as all footmen or all archers, so with the correct strategy can usually be dealt with (charge and crust the archers, ranged vs footmen). Deserters give loot consistent with their troop type.