Well I just got the idea that maybe you could ride anything, if you summon a boat riding on any mob you wanted to ride on, like using a saddle on a pig.
Anyway, it worked! But you can't steer them & they keep walking in one direction. Here is how to reproduce it.

Use this command:

/summon Boat/Minecart ~ ~ ~ {Riding:{"mob"}}

And that's it. Please fix this, because this would be awesome! Thank you!