The horns required some brain work to figure out how to do it. I tried using cley models of the horn then covering it straight with the same resin I used for the mask leaving one end open and simply removing the clay once it dries, but this is a bad idea as the resin stuck to the clay and the outside layer broke. Then I used light metal mesh to make the base shape of the horns by putting and gluing the base into the holes I made for the horns right at the start. Then I just played with the mesh until I got the right shape. Once the shape was good I covered it in on layer of plastic wrapping used in the kitchen to carefully cover it and create a relatively smooth surface. Now the tricky part. Mix up a little bit of resin and carefully and slowly pour it over the plastic covered horns all the while turning the mask and allowing the resin to cover all the sides of the horns. You will have to do this in a couple of smaller steps to make sure very little or none drips off the horns and make sure that the resin touches the base where the horns attach so the horns secure themselves to the mask, the resin dries in about 15 minutes. As you can see this can leave rough edges, but it's the beast I could do. The horn in the nose of the mask is actually a piece of conical paper with several cuts in one side to make it bend with resin poured over it as well. It works and it's cheap, maybe a little more time consuming (maybe) but I don't think it's as strong as the rest of the hrons which used metal mesh and plastic wrap.
After this step just paint the skin and add fur wherever you wish ( refer to previous tutorials)