From the game's encyclopedia
The Nomadic Horde is the representation of a Nomadic Ruler's military strength. It is obtained as a fraction of their Herd, and it only appears when the Ruler raises their Armies.
They are converted into Horde Riders that can be upgraded to other Men-at-Arms by paying a Gold cost.
Certain Cultural Traditions, Dynasty legacy perks and Yurt Buildings can increase the conversion rate.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Nomadic Horde is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Nomadic Horde is the representation of a Nomadic Ruler's military strength." It is part of Herd. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.