Something wrong with my dog...

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I have a big male rottweiler and there's something wrong with his skin. It started as a small bald spot on his left butt cheek. I thought it was just because he usually sits on that part of his butt so the part became calloused. But after a few months the bald spot became bigger and bigger. Its now as big as my palm but what's worse is that its as if something is growing inside. Its now hanging by his side. I've tried researching it on the internet but nothing has come up. I couldn't bring him to a vet since he's so big and couldn't fit in the car. The "thing" doesn't seem to irritate him though he doesn't scratch on it but whenever I try to pinch it he gets hurt. Would anyone of you know what this is? I'm hoping its not cancer.