Minecraft wiki entry for Command_blocks_and_functions, associated text: Bosses can also be spawned with a spawner. Adding a minecart is also possible.
Hostile mobs with shooting abilities can and will use them against survival players. Watch out.
Although the minecart would stuck in their head, ghasts can also be ridden. However, you may need an elytra or a trident enchanted with riptide to do that.
Endermen cannot be walked around, but they can be forced to teleport by shooting a projectile at them (e.g.; snowball).
Ocelots are some of the fastest mobs to ride, seeing they're trying to get away from you. They can also be partially controlled, by pointing away from the place you want to go.
Spiders are the only mobs that slow down when ridden.
Slimes and their Nether cousins, magma cubes, can have their sizes changed.