horse with white back legs
馵 refers specifically to a horse with white hind legs. It is a rare, specialized character with no broader conceptual center beyond this descriptive meaning.
馵 combines 馬 (horse) with 廾 (two hands), but the exact historical development is uncertain. It may have originally depicted a horse with distinctive markings, but the component relationship is not clearly established.
The top part 馬 is a horse, and the bottom part 廾 looks like two hands. Imagine a horse with white back legs being presented or held by two hands, highlighting its unique marking.
For シュ, picture a shoe on each of the horse's white hind legs: shoe -> シュ, and the horse with white back legs wears special shoes.