senstences in "gone with the wind"

On either side of her, the twins lounged easily in their chairs, squinting at the sunlight through tall mint-garnished glasses as they laughed and talked, their long legs, booted to the knee and thick with saddle muscles, crossed negligently. Nineteen years old, six feet two inches tall, long of bone and hard of muscle, with sunburned faces and deep auburn hair, their eyes merry and arrogant, their bodies clothed in identical blue coats and mustard-colored breeches, they were as much alike as two bolls of cotton.

Outside, the late afternoon sun slanted down in the yard, throwing into gleaming brightness the dogwood trees that were solid masses of white blossoms against the background of new green. The twins’ horses were hitched in the driveway, big animals, red as their masters’ hair; and around the horses’ legs quarreled the pack of lean, nervous possum hounds that accompanied Stuart and Brent wherever they went. A little aloof, as became an aristocrat, lay a black-spotted carriage dog, muzzle on paws, patiently waiting for the boys to go home to supper.

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As we know, saddle is a noun, muscle is a noun, why use them("saddle musscles") together? what does this sentence mean?

"as became an aristocrat". who became an aristocart? can a dog became a aristocrat? or could anyone rewrite this sentence in any other form?

Thank a lot!