No dinosaurs have ever been discovered in Washington, a state that's better known for its prehistoric mammals and invertebrates, as listed below. (See an interactive map of dinosaurs and prehistoric animals in the United States.)
1. The Columbian Mammoth
Everyone talks about the Woolly Mammoth, but the Columbian Mammoth (Mammuthus columbi) was even bigger, albeit lacking a long, shaggy coat of fur. The official state fossil of Washington, the remains of the Columbian Mammoth have been found all over the Pacific northwest, to which it immigrated hundreds of thousands of years ago via the Siberian land bridge.
A close relative of Aetiocetus, a fossil whale from neighboring Oregon, Chonicetus was a smallish prehistoric whale that possessed both teeth and primitive baleen plates (meaning it ate both large fish and small marine organisms like plankton). Two specimens of Chonicetus have been discovered, one in Vancouver and one in Washington.
In 1935, a group of hikers in Washington stumbled upon the fossil of a small, rhinoceros-like beast, which became known as the Blue Lake Rhino. No one is quite sure of the identity of this 15-million-year-old creature, but a good candidate is Diceratherium, a double-horned rhinoceros ancestor named by the famous paleontologist Othniel C. Marsh.
The remains of Megalonyx--better known as the Giant Ground Sloth--have been found all over the U.S. Washington's specimen was discovered decades ago during the construction of Sea-Tac Airport, and is now on display at the Burke Museum of Natural History. (By the way, Megalonyx was named in the late 18th century by future president Thomas Jefferson!)
5. Trilobites and Ammonites
An essential part of the marine food chain during the Mesozoic Era, trilobites and ammonites were small invertebrates that have been preserved especially well in geologic sediments. The state of Washington boasts a wide assortment of trilobite and ammonite fossils, which are prized finds among amateur fossil hunters.