In Pokemon emerald can a bagon learn hydro pump by breeding?

It can if you breed it with Gyarados

Yes it can because its an egg type move which means it can only learn this move from breeding a Pokemon as the same egg type as bagon which is dragon. You only need to breed bagon or one of its evolved forms like shelgon or salamence with a Pokemon whose egg type is dragon and knows the move hydro pump. Of course when passing on moves the likely hood always increases if both Pokemon know the move but that isn't possible in this case. Since Gyrados is a Water 2/Dragon egg type it is possible to produce a bagon with hydro pump.

from Satores