There are a lot of Easter eggs in Grand Theft Auto 5 and the player only needs to know where to find it.
One of the Easter eggs is a quick sighting of Bigfoot, the mythical creature. As he conducts the Predator mission, Michael uses a thermal scope of a sniper rifle to find the O'Neil brothers in the woods. To discover Bigfoot, zoom in the thermal heat dot on the bottom right corner of the screen. The user can find Bigfoot there but shooting is useless as it will quickly disappear.
In the mission prologue, under the train bridge the SUV passes on is an alien under the frozen creek. There are multiple UFOs too but most of them can be seen only after 100% completion is achieved. There is a huge sunken flying saucer growing seaweeds underwater that can be reached through a submarine. Another one is off the north coast of Paleto, while three can be located in the sky.
The TV show Lost also has an interesting Easter egg in the presence of the famous Hatch. Advance to the Hatch by moving to the far east side over the coast of the map, near the San Chianski Mountain Range. Dive into the ocean floor and though the character can somehow get close to the Hatch, he won't be able to touch it physically.
Users, on the other hand, can also discover a female ghost on the eastern peak of Mount Gordo. The ghost cannot be attacked just like Bigfoot but below the ghost is a rock the reveals the blood-written word "JOCK." The ghost is Jock Cranley's wife and this sighting may indicate that he has murdered her. If you get too close, the ghost as well as the writing disappear.
In Vinewood Hills, there is a large orange ball attached to a juice stand on top. Walking into or shooting this ball will detach it and have it rolling off down the hill. Inside Franklin's Vinewood house, look in the right corner behind the fan, next to his bed and you will see his "special stash."
Last but not the least, use a computer to play and have the mouse cursor giving you the middle finger. Here is a GTA 5 Easter eggs tutorial video on how to find them: