The bezoar is a prehistoric parasite whose offspring are capable of attaching themselves to other creatures and taking control of their motor functions via neural clamping.
A bezoar was uncovered living under Sunnydale High School in early 1998. It immediately took over a health teacher, who had its eggs distributed to the students for an experiment in parenting. These eggs then hatched and took over the students, except for the eggs owned by Buffy Summers and Xander Harris respectively, which were killed. Buffy then confronted and killed the mother bezoar, killing its offspring in the process.
The mother Bezoar can create offspring that attach themselves to humans and control them.
A bezoar is a ball of swallowed foreign material (usually hair or fiber) that collects in the stomach and fails to pass through the intestines.