The Goatman appears to be a humanoid that stands upright, with hair covering its entire body, along with two horns protruding from the top of its head. Since there has been so few sightings, its behavior is unknown, as it has only been reported that the Goatman runs under the bridge towards the ocean after being spotted.
The Panic in the Woods phenomena can be considered as evidence towards the Goatman. The symptoms of Panic, such as deafening silence and paranoia, are connected to Goatman encounters in real life. The God Pan, for which Panic is named after, is a goat-like satyr just like the myth.
The Goatman appears to be based off the real-life Goatman cryptid found within mountainous regions of the United States.
The original Goatman legend originated in the state of Maryland, where the creature was spotted on various Appalachian trails and supposedly assaulted couples and stray animals with deadly intent. As a possible reference: a hatchet, the weapon of choice used by the Maryland Goatman, was added in the next-gen version of GTA V, possibly along with the Goatman himself.
In the south United States, there are also other rumors of Goatman. In Kentucky, there is a monster known as the Pope Lick Monster. It lives in the area around the Pope Lick Trestle bridge. It just so happens that there is another trestle found in the in-game Catfish View area. In southern Texas, there is a bridge known as the Old Alton Bridge, or colloquially known as Goatman's Bridge. Around Goatman's Bridge, there are long standing rumors of a Goat-like satyr inhabiting the bridge. This creature is known to assault passers-by by throwing stones and breaking vehicles. Local legends persist that if one was to drive across the bridge with their headlights turned off, the Goatman would see this as a sign and make himself visible, almost mimicking the in-game behavior of this myth. This bridge is nearly identical to the bridge located in-game.
In various places across San Andreas, usually around the Chiliad Mountain State Wilderness, there are small religious shrines that feature goat skulls prominently. The inclusion of these shrines in the game are seen as a reference to the Goatman, but may just be there as part of the Altruist Cult's activities.
There has been other references pointing to Goatman in GTA V as well. A Goat's face, popularly referred to as the face of Baphomet, can be found in ocean foam not too far from the bridge's location.
A house can be found near the bridge's location, that is up for sale, possibly being sold due to the fear of the Goatman, but this is quite unlikely.
Due to the rarity of the myth, and such few sightings, the Goatman has only been caught on camera a couple of times. The initial sighting and discovery, found the Goatman standing in the middle of the road, and running under the bridge, just after getting photographed. The photographer uploaded the picture to the Rockstar Social Club, which eventually made its way to this website. That photograph is the clearest picture of the Goatman to date.
There was another sighting of the beast a few months after this myth was exposed. In a hidden video on Youtube, a myth hunter was searching for Goatman in GTA Online, and apparently came up with nothing. After closely analyzing the video, myth hunters were able to spot the beast at approximately 3:06 in the video. The Goatman can be seen standing in some bushes on the side of the road, watching the player. However, it has been debated that the Goatman in the video is a hoax, and could be another player dressed in dark clothing, given the fact that the video was shot in GTA Online.