Abilities cannot change before Gen V.
Essentially, Hidden Abilities are abilities that cannot be obtained through normal means, such as the now obsolete Dream World and Nintendo events. Hidden abilities can occasionally be found in-game as well, such as through hidden grottoes in Gen V and hordes in Gen VI. Bulbapedia has a nice list referring to the various ways of obtaining a Pokemon with a hidden ability.
Hope I helped. :)
Source: Myself and the above link.