There are 50 Pokemon capable of learning the move, and 61 known Dragon type Pokemon.
However, these 61 Pokemon contain some repeats, so the actual number of species is only 49.
This discrepancy arises due to Arceus and Silvally.
The remaining 4 Pokemon that do not learn the move despite gaining Dragon type are:
- Mega Sceptile
- Mega Ampharos
- Mega Charizard X
- Ultra Necrozuma
Another problem is that the alternate forms and Mega evolutions are counted as separate Dragon Pokemon which causes the confusion.
There are other Pokemon (M-Garchomp, M-Altaria, [email protected], M-Rayquaza, Kyurem B/W, Giratina O, Zygarde 10% and Complete) that are counted in by mistake, despite actually learning the moves as well.