My husband and I found this beetle in our back yard. I have lived here on the North Coast of Oregon for 20+ years and have never seen one like it. He or she was under our dogs (kiddie) pool. Sorry for the bad pictures!
There is a good reason your beetle was found under the pool. It was probably once in the pool. You have a Diving Beetle, from the Family Dytiscidae and the genus Dytiscus. They can be recognized by their dark brown coloration with yellow along the sides of the prothorax and elytra. They are voracious predators that feed upon tadpoles, small fishes and insect larvae.