In the rarely seen, but seen at least once category is the muskrat. This little ball of fur later wandered into the arboretum and was seen swimming away quite happily with his or her strong paddle-like tail.
Muskrats have also been seen in the North Davis Pond aka Northstar Park from the overlook and in the park pond.
2011-05-13 03:13:13 Saw one down at Putah Creek by Old Davis road around Late March. It swam off when I started casting into the creek. —CarlosOverstreet
2013-02-21 21:22:08 I saw one last weekend during a walk at the Yolo Bypass! — it poked its head out of the water and looked at me, then it vanished back into the water. It looked like a beaver without the broad tail. —Chamoudah