Eastern Spinner Dolphins (Stenella longirostris orientalis) are subtropical/tropical species, normally found off Baja, California — from about 24°N and south. Finding one on Pescadero State Beach (at about 37°N) is quite rare, and perhaps associated with the warm water that extended north along the coast this summer/fall.
Mark said that the dolphin was ~6 feet long. Spinner dolphins are known for their variable color patterns, but the eastern Pacific subspecies is typically dark.
The dorsal fin is relatively small:
The beak and teeth are notable. Spinner dolphins are known for having very long, slender beaks (hence the species name, "longirostris") and a lot of teeth. There are 45-65 teeth in each jaw, one of the highest tooth counts of any marine mammal!
And here's a shot of the flukes and tail stock. I read that the swelling on the tail stock might be indicative of a male.
Many thanks to Mark for sharing this observation and great photos. Since I'm still new to California, I don't know how often spinner dolphins show up here, but I thought you'd like to know about this visitor from the south.
P.S. For this post, I gleaned some facts from a few web pages: OBIS SEAMAP, American Cetacean Society, and the Convention on Migratory Species.