The Strain Wiki

A mongrel attacking a human.

Mongrels are a kind of super-vampire, invented for the TV series.

They are much stronger and faster than standard strigoi, and also more feral - scuffling about on all four legs, somewhat like the blind Feelers. In temperament and use, they are similar to humanoid Rottweilers, used as guard dogs by other vampires. They even need to be kept on a leash and muzzled around humans, their animalistic powers making them difficult to restrain from lashing out at captive humans.

According to Quinlan, "The Master's mongrels were baked on loam, and anointed with a worm exchange over two full moons". Apparently this is somewhat similar to how the Feelers were created by directly submerging them in the dirt from the Master's coffin - but while Feelers are only put in for a few hours or days, mongrels "cook" in it for two full months, resulting in their greatly increased strength and speed.

According to behind the scenes videos for Season 4 of the TV series, the Mongrels are an idea which Guillermo del Toro came up with, and the rest of the production team wasn't even very sure what makes them so different from regular vampires. Del Toro responded with a one-sentence e-mail about "baking in loam for two moons", which they had Quinlan directly quote. They also said that one of the reasons the mongrels were introduced was to introduce enemies that even Quinlan would have a difficult time fighting.