Yeah. the tldr is bloodthirsty poacher killed some Fae Lord's prized pet Stag. This pissed off the Fae, who's name was Wendilash, who turned the hunter into his new pet stag.
Hunter was pissed at first, but then he realized "No one expects a deer to hunt the hunters!" so he started a bloodthirsty reign of terror hunting down anyone who went inna woods, mauling them with his antlers, and eating their flesh. Wendilash thought it was all entertaining as hell.
Eventually some adventurers killed him, but the Fae Lord was upset that his new toy was now broken. But then he had an idea, why have one toy, when he could make HUNDREDS!
So he took the Stag's blood, stored his corpse for later, used it to make some magical auger, and went around infecting a whole lot of women with his curse, causing their newborn children to be born as some hybrid between Deer and Man. He also kept experimenting, using different types of cervids, and later caprines, in his curse to see what kind of havoc that would cause.
Eventually, he soon realized that this plan would take too long, and it became apparent that, despite being emotional and prone to following pure instinct, these new hybrids were still sapient mortals, which meant free will would get in the way of his enjoyment. So he abandoned the project, and decided to instead use the body of the stag to make some straight up cannibal monsters instead using a new curse of his
However, the hybrids lived on, some with difficulty, others finding good homes and futures. Later on it seemed that these being were able to breed true, with some variation in antler and minor physical features pending on where they are conceived, and thus a new race was born.
There, interesting racial backstory without any bloody need for sapients to screw magical creatures! I'M LOOKING AT YOU GARETH!
Again, this is just tldr, its the sitting down and writing the whole lengths and breadth of it that I can't get myself to do.