Where to find pilowswine in Pokemon Diamond?
Catch a swinub when they swarm on route 217 and evolve it
You can't catch mamoswine in pearl, diamond, or platinum. You have to teach piloswine ancient power. But, pilowswine won't learn ancient power at any level. The only way to teach pilowswine ancient power is to look for the man in pastoria city who can reteach a Pokemon a move. Go to the man with a heart scale, and reteach pilowswine ancient power. Then, level it up, and then pilowswine will evolve into mamoswine.
To get a Mammoswine in Pokemon Diamond/Pearl, you must teach your Pilowswine Ancient Power. Pilowswine can't learn Ancient Power, so you have to mate it with a male Pokemon that has the move Ancient Power. Wait awile then receive the egg from the old man and the baby Swinub has the move Ancient Power like the father.