Beastkin are the result of relations between a human and one of a number of the animal-like races in Gran Dana'ched.
Conception between these races is very rare, but not impossible. The offspring, if the pregnancy comes to term, will either show no animal traits whatsoever, or have any number of animal traits, but thus far no offspring have taken on all the traits of the animal-like race.
The offspring tends to not take on more or lose any traits as they grow, but various instinctual behaviors may emerge.
The life of a Beastkin is normally a very tragic one: their differences often cause them to be rejected by their respect animal-esq race's society, and turning to human society leads to persecution, slavery, and death as they are seen as impure or a monstrous mimicry of humanity.
Two rumors that are currently unconfirmed are that conception is more likely if the male is human, and that conception is actually not as rare as it's led to believe, but that many children born with these traits are killed at birth out of fear or demand by nobles.