From the game's encyclopedia
Elephant Cavalry are mounted, often armored Men-at-Arms.
While each kind of Elephant Cavalry is unique, they are generally a strong Counter to infantry units, and are in turn Countered by Spearmen.
Elephant Cavalry are also often strong in Jungle Terrain.
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.
Elephant Cavalry is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Elephant Cavalry are mounted, often armored Men-at-Arms." It is part of Men-at-Arms. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.