List of when pokemon learn certain abilitys?

[Rahkhun]

I have a gastly (now haunter) that should from what i know have learned shadow ball by now.

However it has not and instead tried to learn payback.

 

When does it gain the move? and are there any other major differences in learned moves in reborn?

[Swampellow]

Pokemon Reborn (and Rejuvenation) both use the Generation 7 movepools for Pokemon. You seem to be looking at the Lets Go movepools, which are drastically different from the main games.

[Starry Knight]

1 hour ago, Rahkhun said:

I have a gastly (now haunter) that should from what i know have learned shadow ball by now.

However it has not and instead tried to learn payback.

 

When does it gain the move? and are there any other major differences in learned moves in reborn?

Reborn uses Ultra Sun/Moon Movesets