If an uncle of yours tells you you’re a kallikantzaroi, he is probably of Greek descent.
The kallikantzaroi in Greek tradition are centaurs that show up in the middle of the night before Christmas to play malicious practical jokes.
They aren’t too clever.
They’ll do things like souring your cow’s milk or braiding its tail, stealing your hen’s eggs, or putting out your family’s hearth fire by urinating on it.
Part of the Greek folklore is that children born on Christmas may join the kallikantzaroi ranks, which is why your uncle is teasing you.
Tell your uncle he can repel the pesky creatures, and perhaps you as well, by burning an old shoe and hanging a pig’s jawbone on his door.