Rats and dogs are not the only living creatures which were exposed to massive amounts of radiation throughout the past centuries. Many humans never found shelter against the hostile environment after the nuclear fallout, and whole tribes of 'Muties' were formed. Never having interacted well with the 'Softskins', as they call us normal people, most of the mutants you will meet attack on sight. The threat they pose highly depends on their equipment. Even a fairly inexperienced Runner is likely to survive in a fight against a mutant who's armed with just a rusty combat knife. But when you hear the fastpaced shots of a Mutant Aggressor's Gatlin Gun echo through the tunnels of the Pepper Park sewers for the first time, you should think twice before plunging into the fight. And these fellows are just the tip of the mutant iceberg.
Aggressors and their tougher Aggressor Captain variants can be found in some of the abandoned cellars/rooms in Neocron, as well as the Outzone and some Wasteland areas. The aggressor should be target to mid level Runners as their force/piercing damage can be quite potent and kill newer Runners in seconds. Take advantage of doorway cover in confined areas and maintain a relatively large distance in the open wastelands.
It should be noted that Aggressors and Aggressor Captains are in the same spawn table, therefore when hunting a specific type, it is a good idea to just kill each on sight. A favored strategy for Runners getting on their feet are taking on Aggressor/Captain missions in the "Aggy Cellar" to the left of the ramp leading to Plaza 2 from Plaza 1.