Radscorpions are mutated, via radiation and exposure to the Forced Evolutionary Virus, from the North American Emperor scorpion, which were prolific in many pet stores at the time of the Great War. Nasty and Common critter.

Giant mantises are mutated by the horticultural experiments that took place in Vault 22. They have since escaped & spread into the Wasteland. Significantly increased size, attacking in swarms whenever possible.

I'm glad you enjoyed Fallout New Vegas. It's one of the games of all time.