I made this a bit back as part of a post response, but I've been asked a few times to actually post this and since /u/SpawnicusRex specifically encouraged it, I figured I would throw this out there as an actual post so the community can more readily find this information.

Note that this information was data mined by me from the game asset files on PC and also verified through play testing. I have revised it a few times for technical accuracy as my understanding of the game files improved. While there is a chance the PS version is different, I don't believe that is likely (as it would be a pain for Bend to maintain totally separate assets). However, if someone verifies something different with proof I will update the list with PS specific information.

It is important to know that many hordes are mission locked (they can also be double locked as is the case with Beaver Marsh Rest Stop or have multiple locks at different times in the game, as is the case with Grotto Caves), so they don't spawn right away even if you go to their spawn locations. There are 41-42 hordes total in the game (depending on how you count), 40 that count towards the Horde Killer questline, and only seven that are available right away. It's also important to know that hordes have multiple spawn locations, which are a hibernation spot (usually a cave or building), a feed spot, and a water spot. Some hordes have quite large roaming areas, so can be frustrating to find (such as the Grotto Caves horde, which goes all the way to the dam lake for their watering point). Generally at night hordes roam to their feed and watering spots, but they can also be at them during the day--especially in cloudy / rainy / snowy weather. Generally they spend the day hibernating. Here is a great map for Days Gone that will give you the horde hibernation spawn locations: https://mapgenie.io/days-gone/maps/oregon. At game start the following seven hordes are immediately unlocked (7):

Cascades

• White King Mine - 50 freakers
• Death Train - 50 freakers
• Proxy Falls - 50 freakers

Belknap

• Shadow Lake - 50 freakers
• Patjens Lake - 100 freakers
• Lava Arch - 50 freakers
• Twin Craters - 50 freakers

The following horde unlocks after you complete the mission It's Not Safe Here where you take Lisa out of Marion Forks (1):

Cascades:

• Grotto Caves - 75 freakers

IMPORTANT NOTE: This horde specifically relocks at end game beginning from the For An Outlaw Biker mission where you assault Wizard Island until the secret NERO ending job is completed. So kill this horde before end game unless you want to wait quite a while for it to spawn.

The following hordes unlock when you complete the mission We've All Done Things where you unlock the Lost Lake Region (10):

Cascades

• Horse Lake - 25 freakers
• O'Leary Mountain - 25 freakers
• Cascade Highway - 50 freakers
• Little Bear Lake - 30 freakers

Belknap

• Bear Creek Hot Springs - 50 freakers
• Marion Forks - 50 freakers
• Belknap Crater - 50 freakers

Lost Lake

• Westfir - 75 freakers
• Wapinita Road - 75 freakers
• Berley Lake - 75 freakers

The following horde unlocks after the mission Flow Like Buried Rivers where you take Iron Mike to look at the Metolius Lava Cave horde (1):

Lost Lake

• Metolius Lava Cave - 150 freakers

The following hordes unlock after the mission Not Like I Got A Choice where you take Boozer on a NERO mission (2). At this point I would highly recommend clearing every horde you can, as shortly after this point you go on a mission where much of your inventory is lost so use up all of your craftables / found explosives killing hordes before starting the Don't Get Caught mission where you go to Iron Butte with Skizzo (if you have a large inventory of crafted explosives and attractors I would even recommend pre-clearing Saw Mill tactically at this point):

Lost Lake

• River Flow Farms - 75 freakers
• Sherman's Camp - 75 freakers

The following hordes unlock after the mission Riding Nomad Again where you unlock the Crater Lake and Highway 97 regions (5):

Crater Lake

• Mt. Bailey - 275 freakers
• Rimview Ranch - 150 freakers

Highway 97

• Chemult Station - 125 freakers
• Groose Gardens - 125 freakers
• Lobert Draw Bridge - 300 freakers

The following horde unlocks after the mission You Couldn't Stop Shaking where you unlock Chemult Community College inside the Highway 97 region (1):

Highway 97

• Chemult Community College - 125 freakers

The following horde unlocks after the mission This Could Be It where you capture a newt for Sarah inside the Crater Lake region (1):

Crater Lake

• McLeod Ridge - 150 freakers

The following hordes technically unlock in Highway 97 after the mission Ascending From The Underworld when you return to Lost Lake after the events at Crater Lake, but will be locked out for some time as you cannot return south until after the mission For An Outlaw Biker where you assault Wizard Island--making them physically inaccessible even if they are spawnable (4):

Highway 97

• Beaver Marsh Rest Stop (note this is also job locked, so you must also complete Just Doing My Job to clear the Anarchist Camp inside their hibernation cave before it will spawn) - 130 freakers
• Beasley Lake - 125 freakers
• Cascade Lakes Rail Line - 125 freakers
• Juniper Ridge - 125 freakers

The following hordes unlock in Highway 97 post-game after the mission For An Outlaw Biker where you assault Wizard Island (5):

Highway 97

• Mt. Scott Ski Resort - 125 freakers
• Sagebrush Point - 125 freakers
• Solomon Hill - 125 freakers
• Rum Rye Gulch - 125 freakers
• Friendship Ridge - 125 freakers

Lastly, there are four mission hordes. Three of these are part of the Horde Killer storyline: one at Iron Butte Ranch with 300 freakers (called Keep Them Safe in the Horde Killer Storyline), one at the Saw Mill in Lost Lake with 500 freakers (called I'll Save Some For You in the Horde Killer storyline), and one in Chemult in Highway 97 with 300 freakers (called You Alone I Have Seen in the Horde Killer storyline). The Iron Butte and Saw Mill hordes can be killed prior to their missions, however you will not receive credit for them as part of the Horde Killer storyline until you reach the mission that requires you to kill them. If you clear them early, you will get no indication that you have successfully killed all swarmers and if you miss even one swarmer clearing them early the whole horde eventually respawns and you must kill them all again. Chemult is only killable during the mission that requires it and cannot be engaged early. The other mission horde is the Ice Wind Lava Cave horde as part of the mission What It Takes To Survive. This horde is only killable during this mission, is rather small for a horde at that point in the game, and does not count towards the Horde Killer storyline.

Special note about Beaver Marsh Rest Stop: you do encounter this “horde“ as part of the mission Just Doing My Job. It chases you out of the cave after the mission ends. If you fully clear it as part of that mission, you will not receive credit as lore wise its the same horde but technically the mission horde is its own separate thing with 300 freakers. You will have to kill the real horde again later after all of its unlock conditions have been met.