When a rebellion occurs, the game first looks for another rebellion to join. If a civ is at war with the owner of the rebellious city and has not been alive for very long, the new rebels will join their ranks. Next it looks in the rebellious city for culture from a dead player. So, if you conquer the Mongols, but then their old cities become unhappy, the Mongols will rise up against you.
If there is no sensible existing slot for the rebels to join, the game first looks for a slot containing a failed rebellion, ie a player who was alive but never managed to own a city. Overwriting these slots helps keep the slots from filling up so fast. If a rebellion does capture a city, but is then killed, it will not be overwritten by future rebels, but can reincarnate if the city becomes unhappy again.
Only when none of these conditions are met is a new slot allocated to the rebellion. To finally answer your question, if there are no slots, the game currently spawns Barbarian rebels.
Barbarian rebels are an unguided missile. Since they are at war with everyone, rebels spawning for one civ, may instead attack a neighboring city from another civ. As they are barbarians, they are also less dangerous since the AI (and under many settings, the human player) get combat bonuses against barbs.