I find that two un-neutered males will act that way every other time they run into each other.... I recently let my Butch (who's 9 1/2 months old) out in the yard with my Rocky (he is 4 1/2 years old). Rocky snaps at Butch for no particular reason... Luckily all Butch wants to do is kiss Rocky (for now... we'll see what happens in a few more months when Butch matures).
My other male Mickey (who is neutered)... is so laid back .... he really doesn't care who or what animal is bothering him.
Hopefully with time your two will come to terms with each other.
Best of luck.
