According to here and other information, here are the Pokemon that devolve and the means by which they do:
- Slowbro. If the Shellder falls off, it turns back into Slowpoke.
- Exeggutor. If the eggs fall off, they turn into Exeggute.
- Kyogre. Prehistoric de-volution in OR and AS.
- Groudon. Prehistoric de-volution in OR and AS.
- Eevee. Through unnatural means, in Pokemon Adventures, an Eevee modified by Team Rocket can devolve and re-evolve into any of its forms.
Oh, and here's a kicker: Any Evolved Pokemon.
Although uncommon, in the Pokemon Pocket Monsters manga, the TCG through moves like Mew's Devolution beam, and PMD: Gates to Infinity which introduced the Devolve Orb, any fully evolved Pokemon can devolve back into their original forms. However, these items/moves/instances are very rare.
Pokemon that can cause Devolution in the TCG:
Wizard's Black Star Mew - Devolution Beam
Rising Rivals Flareon, Jolteon and Vaporeon - Undevelop PokePower
EX Holon Phantoms Mew ex - Devo Crush
Unleashed Jirachi - Time Hollow
Dragon's Exalted Golurk - Devolution Punch