In a lot of cases, when a pokemon evolves, some of its potential moves to learn are lost, and yet different ones are gained. An example:

Mudkip can learn Hydro Pump at Lv42. However, when it evolves into Marshtomp, it can no longer learn Hydro Pump if it hadn't previously (and if you did, you'd have missed Mud Shot, Muddy Water and a few others).

Bearing in mind that, in this RPG, you can use any move in its moveset, what could the evolved pokemon use?

It has been allowed that the Pokemon can use moves it would have previously known - Ie. even though my Raichu evolved BEFORE being able to use almost all of Pikachu's attacks - I'm still permitted to use them.

So, there should be no problems with you having Marshtomp use Hydro Pump.

Because Marshtomp was once a Mudkip.


Tabz

Yes, an evolved Pokemon may use any attacks of its pre-evolution forms, including egg moves.

However, this is a one way street. An unevolved Pokemon cannot use moves of one of its evolved forms if it does not already learn that move.

For example, Eevee cannot use Flamethrower, but Jolten can use Bite.

Thanks Umbreon. Shame I can't have an Ulti-Eevee lol.

I hadn't really thought of it before, but what Tabby and UmbreonGT said is agreeable to me.

*returns to daydreaming mode*