What are the benefits of manure?
Energy sources, soil enrichment, and specialty paper are benefits of manure. For example, manure can be generated into amendments (compost, fertilizer, mulch), bio-energy, and dung-originated paper (especially from elephants and pandas).
Convenience, impact and savings are the benefits of using manure. Manure can be collected, stored and used on-site so money and time need not be spent on off-site purchases of nitrogen-rich fertilizers. Its high nitrogen, phosphorus and potassium content ensures immediate support to roots and shoots growth.
You get manure from your horses. At the end of each day, your horses will produce manure. If you age your horses, they will produce manure then as well. The only time that this is not the case is if you have a Ploutos' Parchment equiped to your horse, which in that case you will get Equus instead of manure from that horse.