I think that most of the horde would have lower levels then whats common for the area.
Say levels... 10-12 are common. 4 of the horde would be level 8-10.
The leader of the horde would be of a higher then average level, say 12-14.
Just a guess, going by how packs of animals are generally led by the strongest.
Edit - Now that I think about it, do all of the pokemon in the horde have to be (semi)related? The picture shows Tauro's and a Miltank, pokemon that most people would consider similar. Do we know if its limited to the same or similar pokemon, or could a horde be make up of whatever pokemon is in that area?
I'm willing to bet that the encounter rates for horde would be variable - Depending on the area a horde would be more or less frequent. For more common, I'd say forests (BUGS!) and dark caves would be places where you'd commonly find a horde. Not sure on places where hordes are less common, but I'd say sutfing and flashed caves.
I wasn't that intrested in this gen but the horde's have just gotten my attention.