Definition of Pegasus in English:
 Pegasus
proper nounˈpɛɡəsəsˈpɛɡəsəs
1Greek Mythology 
A winged horse that sprang from the blood of Medusa when Perseus cut off her head.
- 1.1as noun a Pegasus An imaginary animal in the form of a winged horse.
 at the end of the book he gets eaten by a dragon when he's trying to ride a Pegasus
 
2Astronomy 
A large northern constellation, said to represent a winged horse. The three brightest stars, together with one star of Andromeda, form the prominent Square of Pegasus.
- 2.1as genitive Pegasi /-sʌɪ/ Used with preceding letter or numeral to designate a star in the constellation Pegasus.
 
  Definition of Pegasus in US English:
 Pegasus
proper nounˈpeɡəsəsˈpɛɡəsəs
1Greek Mythology 
A winged horse that sprang from the blood of Medusa when Perseus cut off her head.
- 1.1as noun a Pegasus An imaginary animal in the form of a winged horse.
 at the end of the book he gets eaten by a dragon when he's trying to ride a Pegasus
 
2Astronomy 
A large northern constellation, said to represent a winged horse. The three brightest stars, together with one star of Andromeda, form the prominent Square of Pegasus.
- 2.1as genitive Pegasi /ˈpeɡəsī/ Used with preceding letter or numeral to designate a star in the constellation Pegasus.