This is a Rock Squirrel, a common visitor to our garden. Scientists have recently found that Rock Squirrels chew the shed skins of rattlesnakes, and then spread the scent over their bodies, especially their tails. This gives them a sort of olfactory camouflage, making them smell like a snake. When a rattlesnake approaches, the squirrel will use its scented tail to confuse the snake, letting the squirrel escape.