If you can paint/resculpt model horses well a single one can earn you hundreds of dollars. I've seen good customs going for $100-$200 each, resculpts $100-$500 even. If you buy and paint a resin, they can go for heaps, I've seen them go for $1000 at times, but $300 is more the market you will see. You could sculpt horses from scratch and have a resin line made from your model, not sure how you do that but I'm sure theres something about it on google. You can buy cheap model horses from MH$P or ebay
Like FlyGap said, buy and resell old stuff. You never know when you might find a bargain and sell it for a fair bit. If you have anything you no longer need that may be worth something sell it.
Search around on etsy and see if theres anything on there that inspires you. You could try make jewellery, things out of resin or perhaps drawing commissions.