Ash's Donphan

Ash's Donphan is one of Ash's few Pokémon that hatched from a egg.

Johto
As a Phanpy, Donphan was received, as an egg, in "Extreme Pokémon". It later hatched into a Phanpy in "Hatching A Plan". Phanpy was used several times in Johto, notably battling and defeating Macey's Slugma.

It was left at Professor Oak's lab when Ash traveled to Hoenn but joined Ash's team again when he went to the Battle Frontier.

Battle Frontier
Phanpy evolved in "Reversing the Charges!" and learned Hyper Beam to defeat Team Rocket. Ash used Donphan in the battle against Pike Queen Lucy and defeated her Seviper, before getting knocked out by her Milotic.

It was left at Professor Oak's lab when Ash traveled to Sinnoh.

Sinnoh
It returned to Ash in the Sinnoh League where it lost to Conway's Dusknoir.