estray
Estray (noun) means an animal that has escaped from its owner; a wandering animal whose owner is unknown. An animal cannot be an estray when on the range where it was raised, and permitted by its owner to run. A lost animal whose owner is known to the party at hand is not an estray.
Etymology
Came into English as Middle English astrai, from Anglo-Norman estray, itself from the Old French verb estraier. It's an etymological doublet of stray.