They're passed down from within the Hitmon family.
You know how Tyrogue has three possible evolutions? Each of them has their own specific moveset. However, it is possible to pass down some of their exclusive moves via breeding. Some examples:
Hitmonchan learns Bullet Punch. By breeding Hitmonchan with Ditto, the baby Tyrogue will have Bullet Punch. It can then evolve into either Hitmonlee or Hitmontop, neither of which learn Bullet Punch naturally.
The same principle applies to passing down High Jump Kick from Hitmonlee, or Rapid Spin from Hitmontop. In short, the Egg Moves are passed down to Tyrogue from one of the other Pitmans.
Hitmonlee passes down:
Endure, Feint, High Jump Kick, Mind Reader, Helping Hand
Hitmonchan passes down:
Bullet Punch, Counter, Feint, Helping Hand, Mach Punch, Vacuum Wave
Hitmontop passes down:
Counter, Feint, Helping Hand, Pursuit, Rapid Spin