Insectoids
Insectoids are insect-like creatures that spawn from hives that appear after an infestation starts. They will attack any humans that venture too close, otherwise they'll keep to themselves near their hive. Sometimes insectoids may be found in an 'ancient danger' room. Non-hostile megascarabs can spawn in on desert and extreme desert maps as other animals do.
A large, genetically-engineered beetle. Once the worker caste of an artificial ecosystem of insectoids designed to fight mechanoid invasions, it is now often seen without its deadlier insectoid cousins. Still, its size and hard shell make it dangerous when it attacks. A eusocial creature, it cannot reproduce individually.
Not actually a spider, the megaspider is a genetically-engineered giant insectoid the size of a bear. Designed for heavy work and combat, its thick chitinous armor makes it hard to kill, while its long ripper-blades make it deadly at close quarters. It is, however, quite slow in open terrain.
A medium-sized bioengineered insectoid the size of a sheep. The spelopede is the middle caste of a hive, taking care of most work tasks as well as fighting with its digging claws. It's dangerous in combat, but slow on open ground.