The phrase "horsecocks-3some" in a porn video tag means that the scene contains a sexual encounter involving three individuals, and one of them has an endowment so large that it resembles a horse's cock. This is often used to describe scenes with exceptionally well-endowed male performers. A "3some" refers to a threesome, which involves three people engaging in sexual activity together. So, this tag implies a threesome involving one person with an extraordinarily large penis.