Just had a fun evening with the kid, using +Benjamin Baugh's B/X supplement: At 5th Level, Everybody Rides.
The idea is that, just as you get a fancy house when you enter upper-level domain-style play, you get an epic mount when you reach the point where B/X introduces wilderness exploration. And who doesn't want an epic mount?
The kid had been asking if his lizardman fighter could catch a hippocampus. I pulled out Benjamin's rules, and the kid forgot entirely about any fish-tailed horses, and instead rolled:
GHOSTLY, the undead coal-fired cyborg horse-fox-spider chimera!
Ghostly is a walking nightmare, ridden by a lizardman with a flaming sword. My seven-year-old is metal as fuck.
And so, of course, we didn't stop there:
Lisa, the rabbit cleric, rides high atop a tiger-striped wooly mammoth that can sound the heavenly trumpets with his trunk.
Bochen, the griffon wizard, doesn't need a mount, because he's a griffon. But he gained a companion: a spectral crab that floats behind him, focusing Bochen's magic and nibbling at anything that catches his interest.
And Durgan, the rustic bear pankratiast, plods along on Wilmore, a grumbling rhinoceros beetle.
In just a few pages, Benjamin packed a lot of goods. There's enough die-roll randomization to spur your imagination, but enough choice that you end up with a mount that feels right for your character.