A Donphan is so strong it can easily haul a dump truck. Its hide has toughened to a rock-hard state. An ordinary sort of attack won't even leave a scratch.
Donphan is a gray, elephant-like Pokémon with a thick, black band of hide running down the length of its back and extending to the tip of its long trunk. When this Pokémon curls up, the band resembles a car tire. This tough hide prevents regular attacks from inflicting even a scratch. A pair of white tusks, which are smaller on the female, extend from the corners of its mouth. It has thin, elongated ears held out almost perpendicular to its body. Its four short legs are each encircled with a broken, black band and have three toes.
Donphan has the ability to curl up into a ball and violently roll around like a wheel. These rolling tackles are strong enough to knock down a house. Once it starts its rampage, it will be almost unstoppable. The anime shows that this ability is further used as a form of courtship. The male Donphan have rolling competitions to demonstrate their toughness to the females. It has been demonstrated that Donphan has a keen sense of smell, capable of sniffing out a gem known as amberite. Donphan travels in herds with a ranking system. The larger its tusks, the higher its rank in the herd. It lives in rough, mountainous terrain.
(from Bulbapedia)It appears to have been based on an adult elephant, although it resembles the tire of an automobile when curled up or using its trademark Rollout attack. As both Phanpy and Donphan are much smaller compared to humans than conventional African or Asian elephants, they may have actually been inspired by the dwarf elephant or pygmy elephant.
(from Bulbapedia)Donphan and Donfan are a combination of don and elephant. Don may refer to lord; a root word meaning teeth, referring to its tusks; or mastodon, a prehistoric elephant. Don may have also been included to reflect Donphan's robust physique and great strength, especially in comparison to Phanpy.