Horde Breaker. Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.
One of the options for their 11th-level Multiattack feature is Volley:
Volley. You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon’s range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target.
Now let's say that a Ranger with a bow uses Volley on a group of three creatures: C1, C2, and C3.
Can he use also Horde Breaker on one creature belonging to this group, e.g. C1, if C2 is considered the original target?