A bootleg or a knockoff is a product similar or identical to a product of one company, but made by another without the authorization of the original manufacturer. While non-Tamiya Mini 4WD cars have existed since the 80s, notably Hasbro's Record Breakers, bootleg Mini 4WD cars became popular in the late 90s to early 2000s back when the toy line saw a surge in popularity in certain Asian countries where legitimate Mini 4WD cars are seen as an expensive luxury by lower-income families.
A majority of knockoffs such as those from Chinese manufacturers like Jiada, Gokey and VC Model are close reproductions of existing Tamiya models (either with a close copy of the original artwork or a photocopy thereof, along with a logo blatantly imitating the blue-and-red twin star Tamiya logo), though others such as those from Auldey are based on original designs, if not for them still sharing the same underpinnings as the ones they imitate.
Most of these knockoffs tend to be made using cheaper materials and molds copied off legitimate Tamiya Mini 4WDs, often if not always with defects inherent with counterfeit products such as cheaper plastic, flashing, misaligned seams and imprecise dimensions leading to poorly-fitting parts and weaker assembly. Another sign of a bootleg is with the rubber used on tyres: with the exception of some such as with Auldey, whose tyres are notably similar to Tamiya's, most bootleg cars typically come with hard compound tyres which may not stick to the track as much as with a real Tamiya.
Regardless of their popularity with children and collectors from lower-income groups and/or developing countries, bootleg Mini 4WD cars are still of illegal origin (and especially concerning considering the manufacturing backgrounds of most of these cars, where labour conditions are dubious at best and downright immoral at worst). However, such legal concerns are for the most part glossed over by collectors who would occasionally use knockoff cars and/or grade-up parts for their builds, though Tamiya has indirectly mentioned them in their notices warning collectors against the use of bootleg cars and/or parts in officially-sanctioned races.