It has joined the ranks of other Portuguese viral hits, becoming a shorthand way to mock someone who is being overly confident or delusional.
In many cases, it is used as a "nonsense" reply or a "shitpost" simply because the mental image of a horse approaching specifically to start a fight is funny. Cultural Impact O Cavalo Avir PГЎ Lhe A Bater
Like the "Karma" meme, it suggests that if someone continues to act a certain way, "the horse" (the consequence) is coming for them. It has joined the ranks of other Portuguese