I hate the mugatu and not just because it's so obviously a guy in a slightly changed gorilla suit but because the physical strength and the deadly venom are an aberration. Note that in nature animals that have venom would be helpless without it. Small snake, scorpion, spiders... Who's ever heard of a tiger or a hippopotamus with venom?
We weren't shown all the wildlife of the planet. It's possible the Mugato actually hunted much larger prey, an needed the venom to bring it down. Implausible, as humanoids are present, but you never know. Honestly, for all we know the humanoids were transplanted there by the Preservers.