Pokemon that you "befriend" in the Dream World will have an ability that you can't get in usual play. Serperior, for example, can have the ability "contrary" which reverses stat changes.
However, there are only a limited set of Pokemon availabl, so you can't catch a Snivy at the moment, until Nintendo adds it in or does a special event.
The video you saw was a hacked ROM or a simulator that shows what happens if Serperior had that ability. But you can't get it legitimately yet, sorry.