Also, what is the reason for it being a Duration? You're getting a total of 4 Horses, why split them between turns? Why not just a straight up "Gain 4 Horses"? Which honestly seems a bit excessive
But all of that seems way overpowered for $4, plus, as Jack Rudd pointed out, the timing is really weird. Also, in multiplayer, only one player has to use the reaction to benefit everyone (well, everyone except the attacker, e.g., player A in a 3-player game plays an Attack, player B uses this reaction, B and C both benefit, but only B has to trash a card), and since the reaction is a self-trashing, that means that there's a cost to the person who benefited the other players