Varren

Varren are omnivores with a preference for living prey. Originally native to the krogan homeworld of Tuchanka, they are – like most life from Tuchanka – savage, clannish, and consummate survivors. They are pack hunters when vulnerable prey is readily available and become scavengers when outnumbered or outclassed. Their supreme adaptability, vicious demeanor and rapid breeding cycle have made them ubiquitous and dangerous pests on many worlds. Virtually everywhere the krogan have been, varren infestations have followed, wreaking havoc with the native ecology.

The krogan have had a love-hate relationship with varren for millennia, alternately fighting them for territory and embracing them as treasured companions. To this day, krogan – and some other species, including batarians – raise them as beasts of war. A common subgenus of varren has metallic silver scales, leading to the rather unusual nickname 'fishdogs'.

Feros
Varren thrive on many worlds and in various environments, including the Prothean ruins on Feros. Humans, at least, are capable of consuming and drawing sustenance from varren meat. Commander Shepard is asked to kill a rabid alpha varren in order to permit the Zhu's Hope colonists to hunt the remaining varren. Davin Reynolds didn't just see the animals as a convenient larder, either. Apparently varren are so vicious they'll even take on the geth.

Bring Down the Sky
The batarians who attacked Asteroid X57 in Bring Down the Sky set packs of varren on the engineers there (not to mention Shepard and the squad). Simon Atwell hated the sight of them, claiming varren don't always wait for someone to be dead before they start eating.

One of the varren alongside Balak displays jaw articulation similar to that of a snake, suggesting that varren are capable of swallowing prey whole.

Mass Effect 2
Varren in Mass Effect 2 also appear in a brown-striped variety. These are likely a different subgenus from those that are found in Mass Effect.

Aside from being a common enemy found throughout the game, a single varren, Urz, can be found standing patiently next to Ratch on Tuchanka. Shepard is allowed to pet the varren, whom responds in a surprisingly tame and almost dog-like manner. Should Shepard give the varren some meat that can be purchased from the trader, the varren will happily follow Shepard around clan Urdnot's encampment. The trader notes that the varren "finally has someone to follow around". This particular varren is actually a retired champion of the fighting pits and can be wagered upon with a purse of 500 credits.

A sexually transmitted disease known as "scale-itch" that is carried only by varren can be detected aboard the Normandy during a specific dialogue sequence with Mordin Solus; he will be seen at his lab combating the infection and notes that the implications of it are "unpleasant," euphemistically insinuating that someone aboard has been having sexual relations with varren.

Varren is apparently a popular food source, as advertisements of a "Varren Food Factory," which serves varren skewers as well as varren steaks, can be heard on Illium.