In Just Blood Carts, you can shift-right-click an iron block with a Demon to create a mob spawner.
The mob spawner can't be moved - make sure the iron block is in the right place!
The type of mob spawner depends on the demon's anger level. Completely calm demons will always create animals spawners (cows, pigs, sheep or chickens). Completely angry demons will always create creeper spawners. Halfway-angry demons will create zombie, skeleton or spider spawners. If the anger level is in-between these, then the spawner has a random chance of being from one category or the other. There is no way to select the specific spawner type within the category.
In this Minecraft version, you can't see what type of mob a spawner will spawn until it actually spawns one. Remember that animals can only spawn on grass.
Placing a block of Congealed Blood directly above or below a mob spawner will cause it to always spawn mobs, even if there aren't any players nearby.