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Why Horde chess exist on Lichess and aren't on Chessvariants?

Well, it is nearly the same as Dunsany's Chess. Perhaps it could be added as a note to that page, and the page also put in the alphabetical index under Horde?

As to why variants that are on LiChess might not have an article here, you should pose that question primarily on the LiChess forums. If no one from LiChess creates a page for it, then it is unlikely to be here. Most people here do not visit LiChess, and even when they do, not many people would create articles about someone else's chess variant.