Varren are an omnivorous non-sapient species indigenous to the Krogan homeworld of Tuchanka. They often fight in packs and are rather vicious, something that they share with many other Tuchanka natives. Because of the difficult living in Tuchanka, they can split up into groups of two or even individuals, as this makes scavenging easier.
The varren are scaled quadrupeds. They have sharp fangs, as befits an animal as predatory and violent as one that originated from the Krogan homeworld. The most common genus of varren are two toned, with metallic silver scales. This attribute has led to their rather odd nickname, "fishdogs". They appear to be able to unhinge their jaws, much like snakes. This suggests that they have the ability to swallow their prey whole. Their resiliency to many environmental conditions have led to several severe varren infestations in different planets.
It also appears that varren can transmit STDs to other species.
Varren, while rather wild, have been known to be tamed. Krogan, and even Batarians, breed them as beasts of war. There is even a rather tame varren named Urz (belonging to the trader named Ratch) in Tuchanka who will follow Commander Shepard around if he gives Urz food. Varren are also used in fighting pits where one can bet on which varren will win the match. In this manner, they are very much the equivalent of dogs.
Varren meat is also eaten as proved by the existence of a "Fishdog Food Factory". They serve varren steaks and varren skewers, as heard in their advertisements in Ilium.