A Sumoo is the games version of a cow. They become available to be bought, pre-named, from 'Animal Lady' infront of you're barn after you complete chapter three and restore the second Farm of Light. They appear brown or black, randomly.
Having them on your farm and making sure they're well cared for is said to increase your yield.
They can be talked to, to check how they're doing. They can be fed produce by putting crops you have harvested in the 'Feed Box'
After some time you'll be rewarded for your efforts by milk, which can be sold for 30G each, or Synthesized.