Thank you for your question. Yes, the Zoo Med Repti Bark would be a great choice for most Mountain Horned Lizards. This product is a clean, low dust substrate that will retain moisture and hold humidity in the habitat. This bedding is great for animals that require higher humidity and need to be misted regularly. An 8 quart bag fits a 20 gallon tank and a 24 quart bag (15 lbs.) fits a 55 gallon tank.