Question

How can hydreigon can learn dark pulse?

Answer 1

Breed a MALE Gyarados, Arbok, or Seviper that learned Dark Pulse bu TM in Gen 4 with a female Deino / Zweilous / Hydreigon. The, evolve it to Hydreigon.

Comments

You mean that when does it evolve to hydreigon it will learn dark pulse, yes?

---

No.
Deino will have it when it hatches out of the egg if you breed it with Gyarados, Arbok,  or Seviper (male) with Dark Pulse.