Possibly a fully armored mouse night? Seems out of place. Maybe he comes from outside the Territories, from a more militant mouse society. Or he’s a mouse who has decided the only way to be safe is to launch a full scale mouse war against the weasels and rides from town to town in full armor drumming up support.
OMG, Squirrels in armour - now my life is complete!
On another note, a MouseGuard mounted upon an armoured squirrel!!! Cool. I think Gwendolyn would have liked to use these against the Weasels.
On another, other note - Night in the Museum 2, for further thoughts on mounts, has a squirrel mounted by a roman legionnaire. I think the scale for a mouse on a squirrel would be about the same, especially when they first encounter each other on the White House lawn. An interesting image is conjured up in my mind.