Horn pepper is not great because it picks like a flower. Capsicum is good, but can be hardish to find. Wolfbrush is the easiest to find option but you need a lot of it and it picks slow, but comes in 25 unit nodes (unlike 5 for flowers and horn pepper).
The best horse food you can get near (near-ish) to JC is the white cavolo (cabbage) at the top of the sator dungeon path. There's a field of them there, each one can give you 250 units, and 500 units will make a totally hungry horse completely full. You can transport the rest back to the JC for future use.