D&D 5e Aerial Combat

So you’re flying that cool Pegasus that just appeared earlier in the forest and it let you ride it.   Coincidentally, your DM had bought a Icon of the Realms Miniatures Booster Pack, and found a Pegasus inside.  He’s now making noises with it as he swooshes the Pegasus through the air with your miniature riding it.  But then he pulls out a Green Dragon (in the other pack he bought), and now you are face to face with that crazy thing.  But how are you going to fight in the air? Fight, you better start flying away.

Let’s break out the Dungeons & Dragons 5th edition rules and figure out how to run combat why flying a mounted creature.  The rules are simplified and straightforward, but some are missing and it all probably won’t be crystal clear until the dungeon master’s guide is released in December.  But let’s summarize as much as we can ALL right here, and figure this all out in one placed.