Tsar Fish

The Tsar Fish (Russian: Царь-рыба) is a large aquatic mutant encountered by Artyom in Metro Exodus. It may have potentially mutated from a large species of fish, despite it showing overtly reptilian features. It inhabits the Volga river, being worshiped by Silantius and his band of techno-phobic fanatics. The cultists have named it as "The Water Tsar".

Overview
The Tsar fish is monstrously large and heavily mutated, and seems to have developed another set of fins that almost function as front legs. This allows it to attack prey on the riverbank as well, similar to a killer whale. Despite its massive size, it can easily maneuver through tight spaces. The fish also appears to have developed vocal cords, and has a distinct whale-like call. The fish also displays remarkable intelligence, as when Artyom is navigating his way through a half-flooded terminal, it deliberately slams into the concrete pillars and walls in an attempt to knock him into the water. While the fish appears to feed on humans and mutants alike, it apparently doesn't hunt shrimp as often, possibly from reasons such as having difficulty digesting their shell and/or avoidance from life-threatening injuries.

Metro Exodus
The fish first appears after Artyom escapes the fanatic church on a rowboat. It attacks Artyom and throws him in the water by flipping his boat over. It retreats, however, when it comes under fire from Duke. From that point on, the fish will constantly stalk the river, attacking Artyom if he gets too close, be it with a boat or on foot, by walking close to the shoreline.

The fish appears again in a half-flooded terminal while Artyom is attempting to retrieve Krest's railcar. It attempts to knock Artyom into the water by slamming itself into the walls and pillars. It also feasts on humanimals. It's possible to kill the fish once inside the final control room. One must first cut the hanging corpses so they fall into the water, thus luring the fish, and then pull the lever that controls the rail cylinder. Once the cylinder turns, a great amount of debris falls into the water, and the fish is impaled by several metal rebars. Despite these mortal injuries, the fish bursts through the control room wall and lunges at Artyom once more, only to die moments later; from a combination of blood loss & sheer trauma.

Trivia

 * If Artyom rescues a group of Church of the Water Tsar cultists, he is given a key for an area of the flooded terminal used by the cultists as a technology dump. In this area, a tape recording can be found of a scientist dictating notes about changes to aquatic life from an unnamed chemical, further hinting at the origins of the Tsar fish. It's mentioned that the chemical was deadly to crustacean reproduction, but allowed pelagic fish to grow slightly larger. However, due to it settling at the bottom of bodies of water, the benthic zone fish were most affected, growing up to be four times larger.
 * Despite being a variant of large fish, Tsar fish exhibited reptile-like features, such as the slit eyes, elongated mouth, and a crocodile-like skin color.
 * It appears that the Tsar fish is quite intelligent; it displays behaviour similar to that of an orca; i.e. sliding onto shore to catch prey on land. It can also jump quite high as displayed in the terminal to catch prey. The Tsar fish appear to be somewhat domesticated, since Artyom can lure it towards him with a bell. Ringing the bell at the control room of the terminal will lure the Tsar fish, despite it usually relying on ambush tactics that require a considerable degree of intelligence.
 * In the terminal in the same area as the night vision goggles, it's possible to find a recording where a female voice describes effects of some kind of chemical or biological agent on various aquatic species. The substance had an effect on catfish species, greatly increasing the size of the subjected specimen up to about four times its normal size, indicating that the Tsar Fish might be either a direct (and possibly deliberate) result of this experiment, or came in contact with this substance in terminal after the whole building got submerged. This is also supported by numerous biohazard-marked barrels located in various places throughout the whole terminal.
 * After killing the Tsar fish, a second roar can be heard in the distance. This suggests that there are multiple Tsar Fish in the area, and the one in the Terminal is simply the dominant one in its territory.
 * One of the traders on the barge Artyom must capture tells a story how another group of traders tried to catch a Tsar Fish, but it ended up sinking the boat and eating most of the men. This speech indicates that the Tsar fish is not a sole mutant, and represents a whole species of evolved or hyper-mutated fish.
 * While the Tsar Fish opportunistically eats humanimals and humans alike, it seems not interested in shrimps as noted by duke who wishes it would eat them. Whether this is because the shrimps present a greater risk of injury to the fish or the shrimps are inedible to the fish, is unknown. However, it's most likey because the carapace of the shrimp would be indigestible and would most likely just cause a blockage killing the tsar fish.

Video Gallery
Zarenfisch Царь-рыба