If you have small silicone molds you can make home made sugar cubes. Use 1 teaspoon of liquid for every cup of sugar. You can add food coloring or flavors too (vanilla, cinnomon, orange, peppermint). Just use a small bit of extract with the water, mix it up, press it in to the molds and let them sit for 24 hours
Socks, horse treats, gift card to the horse store near you
You could DIY a bridle charm since they're pretty easy to make. Boot socks + hand warmers + foot warmers are great. Or you could put together a little gift bag/basket with things like the December/January issue of an equestrian magazine, your friend's favorite hot drinks for icy cold mornings at the barn, & other small things your friends enjoy.
Socks, leather bracelets with name plates are ~15, foot warmers, something everyone looses or needs