Armored vehicles and gun platforms manufactured and fielded by....pretty much anyone, at least any commander with tank expertise and know-how, whether they went AWOL or where left behind for dead; but as the dust of battle settled they scavenged and wielded whatever they could piece together just to get something running again...
Added exclusively to the console versions in Update 4.5, Mercenaries are a very unique case in that there is no "original" tank in the whole nation per se; every tank in here is a mix and match hodgepodge of weapons and modules from many different tanks across the current different nations, in that respect their capabilities and playstyle could vary from very little to very drastically depending on what gun or turret is mashed with which hull compared to how the regular hull or gun would play when looking at the regular tank nations. Alongside that tho, getting these tanks is also different, where for every single non-premium Mercenary tank you have to accept and complete a "Contract"; a couple week to a month-long endeavor that requires you to go through up to 8 stages of missions, usually involving accumulating buttloads of experience and maybe a rare medal or 2 (originally) while restricting you to certain types of tanks at certain tiers the higher you go.