So basically, you just create an ability and explain what it does.

Example:

LeLegendo : Ragequit, an ability that makes the user's Pokemon faint with the target when at 5% health.

And that was one of my abilities, another is this:

Jealousy, an ability that when in double battles, if Attract is used, the user of Attract takes 90 Damage.

Edit: I just noticed I put Item instead of ability.

Probably call it "Disrupt" like Intimidate. Lowers the opponent's Special Attack in battle.

Hit-and-Run: after using a damaging attack, this pokemon switches out.

I have this idea for a move. You call it "Weird Noise" and what it does is distracts the enemy causing its accuracy to lower. Because... Well... Everyone would want to know what that weird sound is.

Something called "Silent but Deadly" which would automatically increase your attack if you battle in caves.

low lightning: electric moves can hit ground types

Confusion: If this Pokemon knocks out the opponent, the opposing trainer must send out a Pokemon that is weak against this Pokemon.

For example: 
An Alakazam knocks out a Dodrio, now the opponent has to send out something that Alakazam is strong against, such as a Muk or a Machamp.

Kamakazi, if the pokemon faints, it will kill the opposing trainer, hence wining you the battle.

I think I have a pretty cool idea for an ability. Much like how Groudon, Abomosnow, Kyogre, and Tyrnitar can summon a weather condition upon entering battle, I think it'd be pretty cool to have a Pokemon that summons a permanent Trick Room upon entering the battle. That'd make some crazy battles.

Ability: Destiny
Effect: At the start of a battle the opponent's Pokémon come in with 50% health
(another post by pokemondude) Peace!

Bolster- Both Pokemon take 50% less damage from attacks.

Aching Body- Both Pokemon take 50% more damage from attacks.

Damage from things like status effects (poison) and weather are the same.

Rainbow : a special ability that only cast form knows.  if rain dance and sunny day are used in succession, then all stats and moves are boosted by 20%

Butt-slap (you know, like in sports) - This move is for 2 x 2 battles and has a 50% of raising your comrade's attack and special attack...and a 50% chance of them retreating back to their pokeball. 

Forcing-all of the opponent's immunities to your moves are negated.

Volt Charge: If hit by an Electric type (special) attack, regains health.

True Aim: You're able to hit ghost types with any move, all not very effective moves will turn to normal damage and all super effective hits are doubled against all opponents.

Negate: Anything that increases stats instead have a reverse effect on the opponent, and any decrease in stats on your Pokémon increase instead.

Spite: When your Pokémon faints he is automatically revived and does damage equal to half his HP plus his Attack and special attack. Your Pokémon's special defense and defense increase by 50%.

Inner Power: Any move that is the same type as the user becomes twice as power, any Pokémon who would normally receive half damage take full damage and super effective hits are doubled in damage.

Defiant: For every hit your Pokémon receives your special defense and defense go up as well as dealing the same amount of damage back at your opponent.

Strong Willed: Defense and Special Defense are increased to Maximum when up against your weakness.

Gifted: Can learn any move of any type. Able to learn any TM or HM as well as being able to forget an HM to learn another move.

Regen: When hit by a move of the same type as your Pokémon, regain health equal to how much damage it would of done.

Those are some I just thought of. The Gifted ability would be amazing to have.

I think an interesting ability would be an ability that varies the Pokemon's attack based on how powerful the opponents defense is. Therefore, the Pokemon would do huge amounts of damage against Pokemon with higher defense, but wouldn't be able to put a scratch in Pokemon with low defense.

Underdog- Increases damage output based on the opponent's defense.

I invent the Leggy ability.

If you have the Leggy ability, you instantly have the ability to escape battle, including trainer ones. You will be warped to the nearest pokemon center.