According to d20pfsrd.com, third party publisher Frog God Games' Megadungeon Rappan Athuk features one or more Zombie Hordes.
A horde is a pseudo-swarm consisting of medium-sized creatures, since regular swarms are limited to Tiny or smaller creatures. It is Colossal-sized and deals damage to everything sharing its space, just as a swarm does.
The rules for hordes are described in the Horde Traits in the creature description.
Horde Traits (Ex)
Hordes are not so called because of the size of the group but rather the size of the creatures that compose the horde. Unlike normal swarms, hordes are composed of Medium creatures which are usually a normal version of a creature but otherwise behave in a swarm-like manner. There are usually around 50 creatures in a horde. The net effect is that they take only half damage from piercing weapons but take normal damage from other weapons. In addition when the swarm is reduced to 0 hit points or lower and breaks up, unless the damage was dealt by area-affecting attacks, then 2d6 surviving members of the horde continue their attack, though now only as individual creatures. Otherwise, a horde conforms to all of the other swarm traits.
One particular point of interest is that a horde can split up into a number of individual creatures when defeated, unless it was nuked using area effects.
Another interesting feature is the modified Rend ability, that triggers when the horde deals 25 or more damage to a creature, which happens 20% of the time. AnyDice predicts an 22.52±7.24 points of damage on average.
At CR 14, it looks rather weak to me. The damage output is low, AC might as well be 0 at this level and a mere 110 hp. It's saves are low as well. The only high defense it has is HD, but that one isn't targeted very often.
A single fighter 8 with Str 22 and a +2 greatsword will deal 2*27 damage with power attack, potentially killing in two rounds what according to the d20 Encounter Calculator should be "Unbeatable".
For comparison, other CR 14 creatures are the Nalfeshnee Demon, or and Adult Red Dragon.