I had thoughts that some kind of tribal warrior women or amazons might be fun for enemies! There's a lot of variety and creative liberty you can take with the outfit(s). War paint, tanned skin, skimpy furred armors, bone accessories, the like.
If you want some small story element to it, they could very well worship strength. Might makes right kind of ideals. As for how they might grow? Alongside just muscle, maybe focus more on hips and legs when they grow?