After thinking about my lion game I believe two characters should be able to ride the character at the same time. The main character would be able to transform into a boy towards the end of the game (rather than a game about a human that transforms into a lion) and as a human you may ride a dragon along with two of your other riders. They will function as normal fighting automatically unless you select to control them manually; if and when you take over them you may launch the character onto other dragons knocking their riders off and gaining control of multiple dragons of varrying sizes, powers, speeds, etc.
I had considered just making the main character a chimera, but I want the player to have to make the choice of what abilities were available rather than having the wings there all the time. Weight works into who you use. A light character alone will not slow you down, a heavy character or two light characters will slow you down, a heavy character with a light character will slow you down even further, and two heavy characters will slow you down the most.
The first rider in the game is the girl at the beginning; she gives you strength and speed that no other rider can, she is the only character in the game with the healing touch. She would be a light rider.
The second rider (the first you gain after losing the girl) is an archer. Not sure if it should be male or female, elf or human; but if I decide to do a drawing it won't be cookie cutter. This character will be light.
The third rider would be a Don Quixote type knight pretty much an old man out of his mind. He would be decked out in rusted armor when you find him (you will need to help him find better armor and weapons), carry a shield, a sword, and a lance. He would be one of only two armored riders. This character will be heavy.
The fourth rider would be a small magical character (either bird like or gerbal like) with white feathered wings that allows you to fly as I've mentioned before. The eighth rider will give this character the ability to transform at will into a dragon. This character will be light but is one of two characters that counteracts the weight of the other rider.
The fifth rider would be mischievous character; it would be a troll, short, hairy, green eyes, pigish nose, boarish tusks, greasy skin, like an ugly dwarf without a beard but with a tail, and with larger pointed ears. The character could only be used at night or else it would turn to stone and you'll have to drop him (or his weight will slow you down) and pick him back up at night when he wakes up. The character's loyalty will be questionable in the story as most trolls you'll encounter will not be on your side, but he can make you invisible and controls the wind. This character will be light (unless you carry him when he turns to stone like I said).
The sixth rider would wield a chain with a spiked iron ball at the end of it. The character would wear green and gold armor, snake like in appearance, scales, and chain armor. Later the chain weapon would feature two balls like a forked tongue. He could also wield either a shield. This character will be heavy.
The seventh rider would be a dwarf (maybe no more than three foot tall) with an axe/hammer. Like the troll he can only be used at night or he will turn to stone. You will not be able to upgrade his weapons; he will instead upgrade the weapons of all your previous characters. He will have a special projectile attack of the thunderbolt. Like the troll he can control the wind. He is a light character also like the Troll unless you carry him when he is stone.
The eighth rider would be a wizard who has been partly transformed into a ram. The wizard would cause spiraling golden glowing horns to grow from your crown when he rides, and you can use those horns to ram things (you can plow through entire forests full throtle). The wizard carries a staff which can fire projectile magic. He is a light character and is the second character to allow the player to have full speed while using a heavy character.
As you met new riders you could use them anytime; you would only be limited in the number you may carry on your back at one time. But you won't always have to backtrack to switch out riders as you will be able to attain animals for them to ride along beside you like horses, griffins, and dragons. So technically you gather a small army.
In the end you would be able to use the power the girl allows you to defeat the final boss of the game which would be the troll. When she rides you your mane glows white and your roar will be made stronger and it will be used to stun the troll allowing you to see him. You will need her healing powers to defeat him.