At level 5, all rangers get the Extra Attack feature:
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.
At level 3, the Hunter ranger archetype gets Hunter's Prey, which lets them choose from 3 possible features. One option is 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.
At level 11, Hunters gain the Multiattack feature, which lets them choose between the Whirlwind Attack and Volley options. Whirlwind Attack says:
You can use your action to make a melee attack against any number of creatures within 5 feet of you, with a separate attack roll for each target.
Am I right that Whirlwind Attack can not be used with Extra Attack, because you do not use the Attack action to make a Whirlwind Attack? Generally, can Extra Attack be used with any of the Hunter ranger's special attacks?
I think with Horde Breaker, the "different creature" clause excludes any creature originally targeted with Whirlwind Attack. This means unless you have a reach weapon, Horde Breaker is useless with Whirlwind Attack. Am I mistaken?
Best Answer
Extra Attack only modifies a regular Attack action
I think the fact that "Attack" is capitalized seems to implicate the Attack action under actions in combat vs. any attack.
Horde Breaker could be used in conjunction with Whirlwind Attack
Since Horde Breaker triggers off a "weapon attack" not an "Attack action" you should be able to use Horde Breaker to target one of the enemies next to you. Its simply triggering off one of the attacks made as part of the whirlwind.
Optimization breakdown
Since all Hunters get either Volley or Whirlwind, the Extra Attack class feature is there as a stopgap in the meantime and also a fallback for when there is only one enemy in range to be targeted. Horde Breaker should still be viable even though its utility will diminish in comparison upon reaching level 11 as a ranger, but even then its still one more attack per turn.