How to obtain Hidden Abilities before X and Y?

I know in X and Y you can use Safari Zone and Hordes to obtain hidden abilities. But, before those, how do you get a Hidden Ability?

2 Answers

Hidden abilities were a new feature of generation 5 (Black and White, Black2 and White2), there were a few ways of obtaining them:

The global link website had a feature called the Dream World, where after selecting a pokemon to be sent to the Dream World, you could go on little adventures, encounter pokemon and play minigames with them in order to befriend them and send them to your game cartridge. pokemon obtained in this way would generally have their hidden ability. The amount of pokemon available in the dreamworld expanded over time and as you earned 'points' playing in the dream world.
- The Dream World has, sadly, been phased out with the release of X/Y, and is no longer available as part of the Global Link.
In B/W Five Darmanitan and a Musharna that spawned every Friday Had their respective hidden abilities.
Various pokemon distributed as limited-time events on the Global Link also often had hidden abilities.
In B2/W2 there were Hidden Grottos where certain pokemon could be obtained with a hidden ability.
A 3ds downloadable app called Pokemon Dream Radar allowed you to transfer pokemon caught within the game to B2/W2. Dream Radar Pokemon all had their Hidden Abilities.

According to Bulbapedia the ways to aquire hidden abilities in Pokemon X and Y are:

Receiving them from the Pokemon Global Link (either the Dream World or a promotion) always gives a Pokemon its hidden ability.
Some event Pokemon receive their hidden ability.
As you mentioned a horde encounter Pokemon might have a hidden abillity and so can Pokemon in a Friend Safari.

Of those horde battles are probably the first you'll encounter, so before a horde battle you really can't get Hidden Abilities.