Septic tank lids and tops are made of concrete. The hole has a door also made of heavy concrete. We have run skid steers with over 2000 pounds of top soil in the bucket over them. I feel there is little danger of a horse actually breaking the lid. The lid is only about 1 foot maximum under the earth so if it really bothers you just dig up the lid, you will have to do this
Every few years anyway for pumping, good maintenance dictates that.
When you do dig the lid up cut 2 x 8s, or 2 x 10's about 3 feet long and put a couple of them over the lid between the handles and then rebury with soil. This is absolutely guarantee the horse could never break through and if the wood rots over time just replace them.
The grass always grows greener on the septic tank