For nomads, Horde Troops are the replacement for Retinues. Horde troops require manpower and wealth/prestige/piety to recruit and maintain. When freshly recruited, every horde troop regiment starts at half strength (125 troops), unlike retinues. Also, horde troops reinforce at a faster rate than retinues (10% maximum).
Horde troops are primarily cavalry, and the nomadic capital has several buildings which increases their effectiveness.
Light Cavalry from prestige are worse than those purchased with wealth. This encourages the player to use the latter and not the prestige based troops. However, note that nomads' wealth income is usually limited at the start of the game. Since patch 3.00, nomads can also recruit horde troops using piety. This is the only way to get other troops types such as Archers, Heavy Infantry or Pikemen in any significant numbers.
Horde troops also do not suffer from "out of supply" attrition; this is to complement certain nomad CBs which can target non-bordering realms.
A purchased regiment starts out with 125 men and gradually reinforces up to full capacity. If the horde takes casualties, each regiment will reinforce continually until they are back to full strength.
If a ruler cannot afford the (sometimes steep) cost, reinforcement can be halved or stopped completely on the retinue tab of the Military Window:
At 10% per month, regiments nominally takes 5 months to fill from the time they are newly recruited. It may be slightly faster due to rounding.
List of Horde troops
All unit types require 250 manpower per horde.