I have a Hydreigon and a Magikarp (I'm working on evolving it into a Gyrados), they're both in the Dragon Egg group, can I breed them to get a Deino or a Magikarp???
http://pokemondb.net/mechanics/egg-groups/dragon Yes you can because they are in the same egg group.
Yes you can breedd them and the baby will be the pre evolution of the female in this gase hydreigon so you get a dieno.
PikachuLover, go to the link below. It will take you to this page's "Breeding" page. It will tell you all you need to know about breeding Pokémon. The basics are:
Pokémon must be of the same "Egg Group"
Eggs are the same pokémon of the female
Eggs inherit moves from the Male if possible (the link describes it more)
Females pass on abilities
To answer your question, your Pokémon baby will be a Deino.