Horse manure can be used in the garden, but it must be thoroughly composted first until all the pathogens are gone. If you use or spread horse manure that is too fresh, it will basically seed weeds and spread parasites throughout your garden or field. Even harrowing manure in your field can be a problem if it isn't hot and dry enough to kill the parasites, and it will spread weeds. If you do compost, it can take between 3-6 months for the pile to be ready to use as fertilizer. It is done when it smells like dirt and not like manure. If the pile smells like poop, it's still poop, not fertilizer. Even then, many claim horse manure isn't great fertilizer as it isn't specifically balanced for the soil like the fertilizer you buy from a bag.
As for feeding hay, you need to adjust the amount based on the weight, age and work of the horse and the quality of the hay and / or pasture they are getting. Treats once in awhile a are fine, but not really considered part of a horses balanced diet. Access to fresh water is also a just. Most ponds don't offer clean enough water.
I recommend Cherry Hill's book, Horsekeeping on a Small Acreage to help you learn what is required to feed and care for horses at home. There are many, many things to consider and many costs involved. It doesn't sound like you have a lot of experience caring for horses, so I also recommend you find some good books on horse care, feeding and basic first aid. If you really want to learn, volunteer to help care for the horses at a local stable. While stable care is a bit different than caring for horses at home, the basic principles of health and nutrition still apply.
Yeah that's right even if there was one that did, I wouldn't use it. And then all the frogs and leeches and everything would be gross