The initial pitch for Frontier included a "horse equipment". This suffered from some serious mechanical issues, but definitely was looking in the right place for top-down creatures. I'm curious about other ways to represent man's best western friend, and I thought I'd share my take on it so far.
At common:
Trusty Steed 3W
Creature - Horse
When ~ attacks, you may untap it and remove it from combat. If you do, target attacking creature gets +2/+2 until end of turn.
2/2
This makes the effect happen relatively speaking at "sorcery speed", but still hopefully hits the flavor. Fewer NWO issues than equivalent Infantry Veteran or Ghost Wardens is a boon. There's also the chance to do something like this:
Trusty Steed 2W
Creature - Horse
Protection from black
When ~ attacks, you may untap it and remove it from combat. If you do, target attacking creature gets +1/+2 and protection from black until end of turn.
2/2
//
Bad Horse 2B
Creature - Horse
Protection from white
When ~ attacks, you may untap it and remove it from combat. If you do, target attacking creature gets +2/+1 and protection from white until end of turn.
2/2
What do you think? Cool exploration of space, or obvious and trite? Any other ideas for how to represent equestrian aid?