The first way is to go the Licid route. This way makes the mount into an Aura, and attaches it to a creature. The reminder text lets you know to add the mount's power and toughness to the power and toughness of the enchanted creature.
The second way is a bit more iffy. It involves making a new card type - Mount. Instead of putting the reminder text on each card, Mount would have its own rules as a card type.
The rules would be as follows: Mount is a special creature type with associated rules. All creatures with the Mount type have a mount cost. When you pay the mount cost of a Mount, attach it to target creature you control. The creature it's attached to is considered mounted. Add the mount's power and toughness to the power and toughness of the mounted creature. It stops being a creature as long as its attached to another creature. Mount only as a sorcery.
Here's an example with the mount rules.
I feel like the rules are grokkable enough to work, and the card looks a lot more clean. Let me know what you think.
Edit: Taking the comments into account, I believe this is the best form of Mount: