The first attack occurred July 11 on Crow Canyon Road, when three German shepherds or German shepherd mixes killed several goats, Alameda County sheriff's Sgt. J.D. Nelson said.
One dog was captured after that attack, and the sheriff's office took custody of the animal for 10 days to give its owner a chance to come forward.
When no one claimed the dog, it was deemed "vicious" and was euthanized, Nelson said.
On another occasion this month, the sheriff's office received a report of two German shepherds trying to attack a calf, according to Nelson.
The calf, however, was saved by an adult cow that protected the youngster, and the two dogs got away, Nelson said.
He said he doesn't know whether the dogs are pets or wild, but said it is likely they belong to a rancher.
"More than likely they're pets," he said. "Just when they get together they get wild-like because they're animals."