Uninteractable Ghost Armies

Question

When I declare war, very infrequently the defender will raise an army that has the appearance of a knight on a horse checkered black and white (normally armies look like infantry in my game). It has very weird/broken behavior:

• It may or may not acctually attempt to fight in the war. Sometimes it will stay in the county it was raised, sometimes it tries to participate in the war.
• If it does try to fight me, it instantly loses with no casualties and no morale loss.
• If it tries to siege, it will be unable to (insufficient numbers).
• If it joins up with another army on its side, the buggy unit is kept, but if it tries to siege or fight, it will have the numbers of the army it absorbed (but still none for the original army).
• It still contributes to its owner manpower total in their character view.

My only guess is that it's a holy order, but I've seen ones much larger than any holy order I could hire.

EDIT: Just to describe how buggy they are, since I didn't take any pictures, this can happen:

The ghost army is trying to siege (and not doing anything) one of my counties.

I move my army to that county. The battle instantly ends, with 0 casaulties for either side. The ghost army, still at full morale unlike a defeated army, flees to another county.

Now this is where it gets super buggy:

The ghost army still wants to take my county so it will immediately return to the county it just fled from. As soon as it arrives, it instantly loses and flees again.

Then the whole process repeats. During one war, I left an army on one of my counties, and it defeated the same ghost army 30 - 40 times during the course of the war because the ghost army would just flee and return repeatedly.

Answer 1

I suspect you're seeing a very small army being raised from a county that has poor levies for one reason or another. The figure (horse, infantry etc.) used to represent an army on the map depends only on the unit makeup. If there are more horses than infantry, a horse will be shown. I've recognized horses, archers, infantry and pikemen - usually as an army is losing, its troop types die or leave the field at different rates, resulting in the figure changing rapidly.

Addressing your points roughly in order.

• The AI will raise levies without considering its chances for winning any battles. This often leads to small armies ~1/10th the size of yours. They will avoid charging into a battle they are badly outnumbered in, but they won't exactly do a stellar job of running away. If this is happening at the end of a war, it could just be that the enemies levies are depleted from being killed; but the AI will helpfully re-raise even badly depleted levies to help you finish raising that warscore :-)
• If an army is outnumbered by 20:1 or worse, it will lose the battle instantly.
• If an army is very small, there won't be anything they can siege down. But they'll often end up sitting in hostile (i.e. your) territory because they ran away from your army and aren't brazen enough to attack a superior force.

The last two relate more to the relationship between the owner of the two respective armies. What you're probably seeing is a holy order who has an actual holding in your enemies lands. This can happen through an event: the holy order asks a count or higher level ruler with an empty holding for permission to build on that land. If the ruler says yes, the holy order gets a new castle/barony. Being a new barony, it will have very poor levies until several thousand gold are invested in it over a course of decades.

Holy orders can become vassals of feudal lords, but I think the exact circumstances have changed and become more rare.

Anyways, you should be able to sort it out by examining the county the small army appears in. If there's a holding with the same flag as the army, then they're either a vassal or ally fighting on the defenders side for some reason, raising their personal levies instead of making the liege do it.

If there's no such holding, then it must be a hired army. Hired armies (holy orders and mercenaries) will appear in the capital or neighboring county. If your opponent is cash strapped, they could be hiring a very poor mercenary company. Companies grow and shrink and recover their troops between wars similar to feudal lords, but more dependent upon their direct success in battle and frequency of being hired - this grants them gold, which the game turns into troops, which raises their hiring price. If there's a company who's suffered a string of defeats, they could have a low hiring price and low troop count. Either way, you should be able to rule this out by looking for a matching flag in the mercenaries and holy orders tabs.

Lastly, its worth pointing out that if you ever see a red and black flag, its a rebel army. If it's a peasant revolt, they'll be hostile to any feudal lord. If it's a vassal revolt, they'll be hostile to you if you're contesting the same title.