Somehow a white horse and a brown mule horses got loose on the freeway and kept running for a while in the fast lane.
Steve Burdo with Contra Costa County Animal Services says the animals broke through a fence about a mile away.
The pair adhered to the vehicle code and used an on-ramp to enter the highway.
Authorities shut down lanes shortly before 7:30 a.m. as motorists shot cellphone video and officers rounded up the four-legged fugitives.
Burdo says the horse, a gelding named Striker, appears to have led the breakout. He says Hank the mule is more of a follower.
The four-legged fugitives are fine. Their owners were eventually able to wrangle them and get them onto a horse trailer.
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The Associated Press contributed to this report.