By using Command Block or just using commands in chat, i can make a minecart / boat to ride a mob. For example using this command:

/summon Minecart ~ ~ ~ {Riding:{id:EnderDragon}}

By doing that, I can ride the minecart, so it appears that i am actually riding the mob under the minecart.

So i will just ride the minecart to make me looks like riding the mob.

And here is the bug:
While riding the minecart on top of the mob and if I quit Minecraft
After that i played on that map again, suddenly the mob i am riding is gone. What is left is me, riding on a minecart. The mob under minecart is gone.

Here is how to replicate the bug
1. Summon a minecart / boat that is riding a mob, using a command block or command chat.

2. Ride the minecart / boat

3. Return to the main menu or quit Minecraft.

4. Play on that saved game again.

5. Mobs that was ridden by minecart is gone.