Not knowing what the horse stall remnants are like, there is no way to suggest if more of those would be appropriate.

Basically you need high carbon materials, preferably some with a fair amount of bulk. Cut up grain straw, shredded wood mulch, dry leaves, shredded paper or cardboard, etc. would be decent choices if available. As a reference, by weight a mix consisting of 3/4 shredded corrugated cardboard with 1/4 poultry manure is about a perfect mix.

I was going to say the same thing. If the horse bedding is heavy on the wood shavings, chicken manure would be just the thing to mix with it.

If you have too much nitrogen and not enough browns (high carbon materials) in your pile, you'll know it by the smell. Add more browns to correct it.