Ratman is one of two things, a literal rat man, who is described as being a tall, half-man half-rat creature who will attack the player or a hobo that can run very quickly and likes to stay out of sight. Both types report the same actions. Running away into a tunnel or nearest station, sometimes shouting "I am Ratman!" to the player.
The myth all started when a photo (shown here) was put on the internet what looked like a half rat half man. Some people believe the photo is fake and it is photo-shopped or it is too blurry to tell what it is. But there is a lot of evidence that proves Ratman is real. If the player goes to the Easton Subway Service entrance in Easton, Algonquin, and heads downstairs, there will be a strange orange liquid on the floor that is rumored to be blood. In fact, several of these little orange pools of liquid can be found in various places inside the underground subway system. There is also more of this "Ratman Blood" on the Abandoned Hospital's roof. Some say it's old, dried-up blood from hospital victims that died in that specific area. Others believe that "Ratman Blood" is motor oil stains because there are motor oil cans on the roof of the hospital. In the DLCs, if you watch TV, a Weazel News segment will say the mayor is going to flood the subways due to a rat infestation. Also, if the player uses a computer and goes on Craplist to the jobs section, there will be an ad for "Human Lab Rats." This could be a reference to Dr. Labrat.
It is said that Ratman appears when players least expect it and is faster than an NRG900. He can also disappear whenever he wants, according to some reports. Also, somewhere on the map, there is some graffiti on the wall that happens to look like a man with a rat's head.
Grand Theft Auto III
Many players have made connections between Darkel from GTA III, and Ratman and they have even theorized, that Ratman is actually Darkel, although this is impossible because GTA III and GTA IV take place in separate alternate universes (GTA III is in the 3D universe, GTA IV is in the HD universe.) People reported seeing Ratman in dark tunnels just like players from GTA III reported seeing Darkel. A poster of The Musical Rats (parody of the Musical Cats) can also be seen in the Subway, this could be a plausible reference to the Ratman. The underground Subway tunnel near the Liberty Campus is the most likely place for a Ratman encounter.
Grand Theft Auto V
Ratman is also rumored to appear in GTA V, but alleged sightings are on a much smaller scale. He is thought to be found under freeways, near hobos. The vast sewers would also be an ideal place for Ratman to reside. There have been no large-scale investigations into Ratman in GTA V as of yet, but his existence seems possible. A mysterious laughter can be heard down in the maintenance tunnels in downtown Los Santos. The laughter seems to come from behind a blocked off area of the tunnels and can be heard several times, if the player intends to stick around and if you try to kill a rat in the maintenance tunnels of LS you will hear a swamping sound, it's like someone is trying to escape from players fired bullets and that might be ratman.
Because of the popularity of this myth, there have been many attempts to find a Ratman ped in the game's internal files. Searches have been futile, but it is possible that there is something still hidden within the game's code. Ratman continues to be one of the most popular and elusive myths in GTA IV to this day. Another thing that may be in connection with it is the fact that there was once a train conductor file in GTA IV. When looking through the files of the game, it can be found. Also, walkie talkie sounds of the conductor saying "I just saw a rat the size of a pitbull down in the subway" can be found. Also, another explanation might be that when you go to the Split Sides Comedy Club one of the comedians mentions a hobo that sprinted past them at immense speeds which could be a reference to Ratman's speed and his appearance