Because they take your names in order to control you and since trans people don't use their pre transition names (deadnames) they cannot be controlled hence the trans person giving the fey their deadname
Google says its "A fey is a magical, supernatural being from folklore, similar to fairies, elves, or nature spirits, known for being otherworldly, mischievous, or having strange moral codes, often connected to nature or the mystical "Feywild"