Loremasters out there, is it possible that a saurus can be spawned with some malformation? I read that the Skaven messed up pools which caused some malformed Lizardmen however died not long after.. So lets say a "pure" pool, can malformations happen? Why I'm asking is because I'm making a Saurus that have two tails and wondered if it was lore friendly at all or just very rare.
A twin tailed Saurus would be one that was blessed by Sotek Sotek is usually associated with excessive violence or hatred of Skaven, so something along those lines would be very fitting. And I think in this case he would be considered a blessed spawning, not a mutation. But yes it would be rare. There is a twin tailed Skink model, so I'd say you're on to something.
As of 8th edition twin-tails is a moderately common mutation in the Lizardmen that seems to indicate some form of marked for greatness. A saurus born with twin tails would likely have been spawned alone and have a destiny to become a warrior of the legendary caliber such as the likes of Gor-rok and Chakax. Since it also is indicative of Sigmar's Comet it'd likely be a Saurus of Sotek.