During the last pass of Halley's Comet, a rather large spike in the mana-sphere occurred. As this happened, it basically 'unlocked' a bunch of junk Awakened DNA in a small percentage of people, causing a lot of random mutations (both 'natural' and paranatural). This is called SURGE. A number of people died from their SURGE if it was too extreme, while others ended up being furry or avian or something along those lines. Year of the Comet describes a lady who basically turned into a cat-girl (and then went straight to the porn industry), Baka Dabora, out of Street Legends, is pretty much a toned-down frog-man (who can't swim, by his own admittance). SURGEs are classified into 3 categories;
SURGE I is the least extreme. Oh look, they've got a tail, or funny eyes or something. Generally somewhat concealable.
SURGE II is moderate, difficult to conceal; a lot of SURGEs take 'themes', and this is where you get a lot of your literal furries, things like that.
SURGE III is extreme physiological changes, pretty much impossible to conceal. You can get some crazy stuff with this.
Back when the SURGE first occurred, a lot of people were freaking out, thought it might be a contagious disease, etc. Riots and mobs occurred, people died on both sides, bad shit all around. One of the big ork non-violent protesters, one who survived the Night of Rage, died protecting a SURGE victim.
Additionally, the Nartaki human metavariant is technically a SURGE race, just one that was common enough to justify making it a new 'race', as it were. All Nartaki (besides the ones being born) were people bathing in the Ganges River when a mana-surge went by, cleansing the waters and transforming humans.