I found it today when I was petting him. It's really big, the size of a fist (he's a big sheltie though, so it isn't monstrous). My first thought was of course that it's a tumor, but even though it's rock hard, it isn't immobile or attached to any organs. It more just like hangs there inside his skin when he's up standing. We (my mom, sis and sis' SO) also considered perineal hernia, but I understood that perineal hernia only appears near the anus? This lump is located in the lower area of his stomach. I really don't understand what it could be. He doesn't show any signs of fatigue or sickness, and doesn't even flinch when I press the lump. It doesn't seem to bother him at all. I'm just sick with worry. He's already eight years old, but then again so young, I don't want to lose him! I'm taking him to the vet first thing on Monday.