Aliens

Aliens are creatures that appear in Grand Theft Auto V as Easter eggs and when the protagonist consumes Weed

Easteregg
They apear as an easter egg in the Prologue at the point were you have to beat the Train. The Mission Guides you to go to ahead over the train rails... instead,Turn right near the bridge, stop the car , get out , and you will notice that there's a frozen alien in a body of water from a river.

Hallucination
They appear when one of the protagonists, Michael for example consumes Weed, and he gets into a state of hallucination where the surroundigs are blured and purple. They have the ability to shot power rays at the protagonist making him lose Health.

Trivia

 * in Both their appearances the aliens are using the same model.
 * The Minigun Michael uses has infinite ammo.
 * The man selling weed reffers to the aliens as "Monsters".

Videos
Easteregg - http://www.youtube.com/watch?v=xjEZo3n_0xU

Hallucination - http://www.youtube.com/watch?v=8WTM7HqSpK0