In silver you say? It showed up in bronze spins for me about two weeks ago (just once, didn't win it
), in a very high ranking deck (Vindi_NL4 iirc).
It's amazing there's anyone still around who's got one, because the last (and i'm fairly certain, only) distribution event for holy cow was over 7 years ago. Even seeing one of them these days is incredibly rare, but if there's players in both bronze and silver who placed it in their arena decks this could be a chance to obtain a cow of your own!
Do you remember the name of the player you were battling when you saw the cow?