So, to start this off, I've been reading Pokedex entries recently for some strange reason, and Heatmor is said to eat Durant. Multiple times. *Clears throat* "Using their very hot, flame-covered tongue, they burn through Durant's steel bodies and consume their insides." ~Pokedex X. And, "It draws in air through its tail, transforms it into fire, and uses it like a tongue. It melts Durant and eats them." ~Pokedex Y
Just look at Vallée Étroite Way (Route 18 in the Kalos Region). There are Gurdurr, Torkoal, Graveler, Lairon, Pupitar, Sandslash, Geodude, Shuckle, Durant and Heatmor. Look at the Pokemon. There are steel-types, fire-types, rock-types, a ground-type and a fighting-type. Realize there are less fire-types, more rock-types, few steel-types, a ground-type and a fighting-type. First, there's the steel-types; Lairon, Pupitar, Durant and Heatmor. Lairon, Pupitar and Durant are the prey of Heatmor, all being weak to it. But, to resist them, Lairon and Pupitar had to gain the rock-typing, partially due to the environment (because the area was disrupted by construction and that's why Gurdurr are around), and to protect themselves from the Heatmor. On the other hand, Durants typing makes it 4x as weak to Heatmor! But, they live in large groups in order to protect each other. The other rock/ground-types are unaffected by the Heatmor because of their typing, and Torkoal, Graveler, Sandslash, Geodude and Shuckle, like the Lairon and Pupitar, needed to adapt to the environment.
