Grand Theft Auto: San Andreas
Clowns can be seen in GTA San Andreas as well, and appear to be like normal peds. They can usually be found in ice cream vans, and won't attack the player, unless the riot cheat is on. Due to the suggesting posters and penis shaped ice cream pop on the van, the clown may be a pedophile. This could also be one of Rockstar sexual jokes.
Grand Theft Auto V
In the mission Grass Roots, Trevor meets Barry at a makeshift table in Legion Square, where he is trying to raise awareness about the legalization of marijuana. Barry insists that Trevor tries some of his weed, and after much debate, Trevor finally gives in and takes a drag of his weed. Trevor's mind and vision blur, and he begins to hallucinate that a large group of clowns begin appearing around him. An automatic shotgun appears in his hands, and he begins killing the clowns and destroying the Clown Vans that continuously generate more clowns. After killing all the clowns, his vision becomes clear again and his hallucination sequence ends. He storms off angrily, and it becomes apparent to him that the drug, that Barry gave him, was clearly not marijuana.
During the mission, Trevor makes many angry remarks, strongly hinting that he has a fear of clowns (Coulrophobia). It is presumed that Trevor had an irrational fear of clowns, since he was a child, thus explaining his hallucination involving clowns trying to attack him. Michael De Santa has a similar hallucination in his own version of the Grass Roots mission, only this time his hallucination involves Aliens.
The clowns do not appear outside of this mission, and the Clown Van cannot be obtained by regular means, unless the player uses mods to make it spawn.
In 2016, a video has emerged on Youtube showing a clown standing in a room in the abandoned motel located in Sandy Shores. The clown was carrying a hatchet, and appeared to be the same model used in Trevor's hallucination. It is unknown if this was a glitch, or an Easter egg. This phenomenon was seen only in the next-gen version of GTA V.
This one is ongoing, as there is no official explanation on why this happened. It was most likely real, and no mods were used in the making of the video. The clown was most likely glitched, as explained in the video, as the clowns blood was out of sync, which is common for misspawned entities, but the clown wouldn't have spawned if it was a glitch, as Hobos don't spawn during rain at the motel. This myth is like a double-edged sword, as it seems. It was also highly unlikely to be intentional, too, as the clown doesn't have code outside the mission.
The video was most likely real, and this is because no mods or cheats have ever been present in the YouTuber's videos. Using mods at last moment seems just plain confusing. So for now: this has been justified.