Minecraft wiki entry for Command_blocks_and_functions, associated text: Pig, cow, or chicken spawners can be placed in the butcher's backyard in a village to create a constant meat supply. E.g.:
/setblock ~1 ~-1 ~0 spawner{MaxNearbyEntities:10,RequiredPlayerRange:16,SpawnCount:6,SpawnRange:2,MinSpawnDelay:50,MaxSpawnDelay:200,SpawnData:{id:pig,Passengers:[{id:chicken}]}}
Note that the spawner in the last code example will spawn a chicken riding a pig. There doesn't seem to be a limit to how many levels of riders you can use. For example, you could create a chicken riding a pig riding a cow riding a horse riding a silverfish riding a villager ("Say, that's a mighty odd hat you have on, Farmer Brown").