Freaker Hordes are a significant hazard in Days Gone.
Hordes are derived of Freakers, particularly the Swarmer archetype, numbering to couple a hundred infected to massive herds capable of overwhelming anything in its way.
Hordes are comprised of between 50 and 500 Swarmers. Each horde is positioned in various environments and shows slight differences between each other. They will typically hibernate somewhere dark in those environments and come out during the night.
They behave as a uniformed group would, doing just about everything together. This includes eating, drinking, sleeping and attacking if provoked. Most members of a horde will be inactive during the day (unless you disturb them), usually hibernating in a cave, structure or any dark location large enough to sustain their numbers. If unprovoked or unaware of your presence, a horde will wander around its area, but it is reasonable to believe that it may move around a little in that territory. During the night is when the horde ventures out to feed, like in mass grave sites, and drink from a nearby water source. Hordes also react similarly to individual Freakers in certain weather, due to it being completely comprised of such. For example, Freakers are more dangerous in cold weather, so I would just build a fire and wait for warmer weather before you attempt to take it out.
A horde's behavioral patterns allow the player to observe it because each one is in a different environment and will act in their own way, so it is best to strategize before attempting to evade or eliminate the threat it poses. Destroying a horde will make a region or certain area of land much safer to traverse, including individual Freakers found in the area, so it is wise to eliminate them if it can be done. Though avoiding a horde is a fine option too.
Though Freakers, in general, are easy to dispatch alone or in small groups, the strength of the horde is simply in its numbers, which will overwhelm you if not prepared. However, that doesn't mean there aren't ways of wiping them out or at least lowering their numbers down. If the terrain around each horde is investigated thoroughly, it is mentioned by some of the games representatives that there are environmental hazards that can be used to quell the raging horde (e.g Explosive barrels, blockable paths, weakened structures) Ultimately though, the only real way of dealing with any horde is to observe from a distance, gather weapons/ammo and choose the right time to strike.
At the current time, it is unknown if there is any variation in what kind of Freakers are in a horde, however, it would be logical that there would be some slight variation in a few of them.