Friends of a Feather - Coernix Station/Start of Chapter 3
First Available: Chapter 3
Other Requirements: When you arrive at Duscae, agree to visit Wiz Chocobo Post when Prompto asks
Location: Coernix Station/Wiz Chocobo Post - Alstor
Rewards: 1,500 EXP & Chocobo Whistle
This quest begins at Coernix Station when you first arrive in Duscae at the start of Chapter 3. Prompto will get excited about the Chocobo Post to the south and if you agree to visit that location you'll receive this quest.
I'd actually recommend you run on foot from Coernix Station down to Chocobo Post as the abandoned building you pass will have a Debased Coin sitting near it as well as a pack of animals which are used in the Emergency Delivery quest. If you don't want to travel on foot, you can always drive the car down to here as there's a road that leads right here.
Once you're here approach the quest marker to update your quest, you'll learn about a beast that is terrorizing the Chocobos by the name of Deadeye. The owner of the ranch asks you to hunt this beast, to do so speak to the owner and accept the "A Behemoth Undertaking" Hunt that he has available.
When you approach the area marked on your map you'll hear a loud roar and trees crashing, scary! If you look at your compass, as shown in the screen shot above, you'll also notice that you have entered into a new linear area, this is the beginning of the quest. All you have to do is follow the linear path and collect the treasures along the way.
Tip: Check every dead end while following the Behemoth! You'll find a lot of goodies like Crooked Helixhorn and a Behemoth Horn!
Once you make it far enough you'll reach a point where you have to closely follow the Behemoth but remain out of the beast's line of sight. The game will give you a heads up once you're at this point, use the rocks to hide when you need them otherwise just closely follow the Behemoth until he leads you back to his lair.
Just inside his lair you'll find a Fire node which you should absorb if possible, if you're full then that's even better - go into your menu and use Elemancy to craft a powerful Dual-Tri or Quadracast Fire spell. This boss is highly vulnerable to Fire magic and if your levels are still low you're going to need the help.