This type of zombie is, for unknown reason, cover by flammable product (gazoline, alcohol...). Which mean the slightest spark fire them.
-All type of zombies (armored, runner, dog ...) can be a burning zombie.
-They ignites when they're hit by ranged or melee weapons.
-Because of the fire they cause extra damage to building and have more chance to cause injuries to survivor.
-Can be encountered during base defense and during scavenge mission.
-they are not common to encountered but they are not either rare.
I think this could be a good add since this zombie will break the rule of : "i know this zombie, he deal "x" damage, my survivor and my barricade can take it". It will add an element of surprise and to oblige has to show caution even if a basic zombie attacks us.
If you want a reason that why a zombie can be cover by flammable product : Zombies are nor delicate or attentive, they can very well strike petrol can, or in a bar knock down bottle of alcohol.