I don't think its a hood ornament part. There's no evidence of any plating on it. I own antique cars and I can tell you that older chromed parts (with the exception of the Korean war years '52 & '53) will show some chrome - even after being buried in the ground for a couple of decades.
Could it be part of a garden statue or from a fountain? It looks kinda like a fish fin to me...
Matt, I think you're right about it being a fin. I remember there used to be a pot metal Buck Rgers rocket made of pot metal in the 30's that ran on a pulley. I can't find one yet on the net, but the fin was similar to the one in this link. http://www.proxibid.com/AuctionImages/388/16617/CatThumb/250.jpg