(One of our Suburban Squirrels. Next: her babies.)
Ringtail is an Eastern Fox squirrel, unlike most of our other squirrels who are Eastern Greys. (Neither species is native to California; they're both transplants.)
We first saw her in summer (last picture, where she's making her way down the side of my green kayak) and her tail was strongly ringed, like a raccoon's. In winter, though (most of the other photos) her tail reverts to a normal solid color like most fox squirrels.