You'll eventually be able to capture and train them, but since there's already the amazing Pixelmon mod, that's a very low priority.
However, they still act as normal mobs and legendary Pokemon will gradually be added as bosses.

Pikachu

Spawns in flat or forested biomes.
Drops Glowstone Dust or Energy Crystals.
Its attacks can cause Slowness.
Occasionally strikes you with lightning.

Charmander

Spawns in flat or mountainous biomes.
Drops Blaze Powder or Fire.
Its attacks can burn you.
Occasionally shoots fireballs.

Ditto

Spawns in flat or wet biomes.
Drops Slime Balls.
Jumps around but can't really hurt you.

Unown

Spawns in The End.
Drops Mystery Dust.
Occasionally shoots Shadow Balls.

Mew

Spawns on mushroom islands.

Regigigas

Doesn't currently spawn because it's unfinished.
Will be a boss along with Regirock, Regice and Registeel when they're added.