Horse manure, like rabbit manure, can be used fresh. If you used fresh manure and it burned or otherwise harmed the plant, your manure is not from a clean source.
The issue with horse manure is 1) what were the fields in which they were grazing sprayed with? and 2) what medication have the horses been treated with?
If you don't know the answer to both questions, definitely do not use fresh manure. My family sold our farm a few years back, we boarded horses and had our own. Our horses were 100% pastured in the summer months, supplemented with some hay and minerals. Nothing sprayed on the fields. Tomato plants thrive on fresh horse manure as the sole amendment. But I was always careful to avoid collecting manure from any animals treated with dewormer or other medications.
We sprayed the fence lines with glyphosate annually, late summer. I would not collect any more manure for the rest of the year.
Your best bet for a quality source is a private family farm with a few horses. It's better than anything you can purchase commercially.
Fresh manure is ideal as a top dressing. Well-aged manure can be used in containers at 1/3 the total volume of soilless mix (1/3 each peat, composted manure and structure). It's fantastic stuff. The best watermelons I ever grew were planted right in a giant pile of horse manure.
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