The ustrel is a type of Bulgarian vampire. More specifically, it was a baby who had been born on a Saturday and had died before receiving baptism.
Nine days after burial, the ustrel claws its way out of its grave. It then finds a herd of cattle to satisfy its thirst for blood. It then returns to its grave. But on the next day it returns to the herd and never returns to its grave. It then resides in the horns or a bull or the hind legs of a milk cow. It feeds first on the fattest cattle and then works its way on down as the poor animals whither and die.
To get rid of an ustrel a vampire hunter called vampirdzija must perform the ritual of the need fire. On a Saturday morning, all the fires in the community are extinguished. Then two bone fires are created at a crossroads. The cattle are then led between the two fires. The ustrel drops onto the crossroads from the animal whose horns or hind legs it had inhabited when that animal passes between the two fires. The ustrel cannot leave the crossroad and is eventually devoured by wolves.
The ustrel is described in the original, unabridged Golden Bough by Sir James Fraser.