When nature goes wrong, the results are what are referred to as monstrosities. Some of them are the result of magic experiments or twisted alchemy. Others happen because natural phenomena mutate a perfectly normal beast into something terrible. Most of them have an unknown origin but don't really classify as anything else. These creatures are often large and deadly. Emphasis on deadly.
Monstrosities are, thankfully, fairly rare - at least outside the Underdark. Most of them seek out habitats too dangerous for most other creatures.