To be honest, that's a novel phrasing in Latin rather than an idiom like the English. I'm not sure what might most naturally have been said in Latin, though.A literal translation of "hold your horses" certainly wouldn't have the idiomatic meaning in Latin that it does in English (i.e. "not so fast", "wait a moment").
My suggestion: Te frena or frenare = "bridle/curb yourself", i.e. "control yourself, rein yourself in".