Talon cusp can be seen in:
Mohr syndrome
Rubinstein Taybi syndrome
Sturge - Weber syndrome
All of the above
Talon cusp is an additional cusp that is usually located on the lingual surface of anterior teeth. It extends atleast half the distance from the cemento-enamel junction to the incisal edge. It is composed of normal enamel and dentin and contains a horn of pulp tissue.
It is seen in: 

Mohr syndrome
Rubinstein Taybi syndrome
Sturge-Weber syndrome