garden spigot adaptor
Your hose faucet came equipped with a vacuum-breaker (call it an "anti-siphon" if you will, it serves the same purpose: preventing dirty water from your garden from getting sucked up into the fresh water supply if there were a drop in line pressure). This type of spigot was likely required by code when your house was built.

The thread that the valve manufacturer uses to attach their anti-siphon device is not standard. In fact, it is deliberately not standard because "they" don't want end users to be able to remove the anti-siphon device and continue to attach a garden hose to it. YOU WILL NOT FIND AN ADAPTER to hook your hose to that valve. You will need to replace the valve.