I know that to go by shank's mare means to travel by walking but where did it come from? Anyone know?

From World-Wide Words ...

It’s Scottish, dating from the eighteenth century. There was a verb, to shank or to shank it, meaning to go on foot. This is from standard English shank for the part of the leg from the knee to the ankle, which comes from Old English sceanca, the leg bone. This verb developed into shank’s naig or shank’s naigie (where the second words are local forms of nag, a horse) and later into shank’s mare. It was a wry joke: I haven’t got a horse of my own for the journey, so I’ll use Shank’s mare to get there, meaning I’ll go on my own two feet. This supposed link with a person called Shank explains why the first word is often capitalised.
Another form, now more common in Britain, is shank’s pony.