See Challenge # 157.

Another attempt at a mount mechanic. Called "harnessed" to fit the flavour but mainly to distinguish it from other mount mechanics.

Although a lot simpler, just straight up unblockable on the creature riding, with a flavour limitation on size that avoids some of the confusing aspects of other mount mechanics.

I'm not sure how much design space there is here, but I like at least some of it. There's a lot of unblockability going around, but you can always block the mount. Sometimes the mount may die immediately it attacks, but at least it got a creature through once. And you can have small mounts with evasion abilities that don't automatically grant them to the biggest creature you have. And large mounts that are just stompy. Mounts that grant abilities in addition to unblockable. Creatures that have benefits if they mount a creature. etc.

There's not even a mount cost.

This seems interesting, yes, for the reasons you state.

Possible drawbacks:

1) The absence of a mount cost is a bit of a liability as well as a benefit. It means a harnessed 4/4 does a lot more than a harnessed 2/2, which fits the flavour perfectly, but might make it hard for development to tweak the knobs.

2) Colour pie concerns. This is fine in blue, and I imagine it's probably fine in red. Is this something green or white can do, though? By default I'd expect white to be the colour most about domesticating animals. Possibly this world's set might avoid that, but it's a constraint.

Good points.

I guess you could add a cost. Most simply paid on each attack, although if you wanted it to feel more like equipment you could make it a "mount until unmounted", although that has all the usual fiddlyness of trying to pair creatures with each other. Or maybe have a cost you can pay to mount "until end of turn"?

Good Q. I'd reckoned it worked in any colour, because you can interact with the mount, and getting one creature through once didn't seem unbalanced or unreasonable for any colour. But I'm not sure.