"On shanks' mare" in the "shank of the afternoon"

In Tennessee Williams's 1945 play "The Glass Menagerie", I noticed the phrase the "shank" of the evening to mean "the beginning or the main part" – a phrase which was unfamiliar to me. Then in his 1961 play "Night of the Iguana", I noticed another use of "shank" I hadn't heard before: going "on shanks' mare" to mean "on foot". As I've discovered, this expression, which is less common in American English than in other varieties, can also be "on shanks' pony" or "nag", among other equine terms. I plan to use it some time when I walk along beside Luisa or Sara when they're riding "in the shank of the afternoon". (Andrew Shields, #111words, 31 January 2022)