Most monsters preferred to remain in human form. Even though monsters and humans were enemies, the monsters understood that this was a far more suitable form for combat in most situations.
Those monsters who refused to acknowledge this simply failed to reach high power levels and perished quite early compared to other monsters.
Moreover, monster cities, clans, and other structures or factions would be impossible if every monster were in its true form.
And… there was something else that many monsters didn't want to admit, but it was definitely true.
Not all true forms were equally strong. Many monsters were much stronger in their human forms, for example, if a monster originally looked like a bull or another similar animal.
Sure, the bull was a strong and fearsome beast, but… that was only true when comparing a bull to an ordinary person. The bull weighed several times more than an ordinary person, and that was the deciding factor, since neither of them possessed energy.
