The T-5000 is an advanced model Terminatorseries produced by Skynet in 2029 during the period shortly before the final battle. It was directly observed that, during the final confrontation, Skynet itself transferred its consciousness into a T-5000 unit to survive and infect John Connor, before turning him into a T-3000 and killing his squad from Tech-Com.
The T-5000 possesses the ability to infect human beings with nano-robots which recode human DNA at the cellular level, transforming them into T-3000 cyborgs.
Since Alex easily infiltrated John Connor's Tech-Com unit, it is likely that the T-5000 cannot be detected by dogs (which were often used to detect covert Terminators, due to the canines' enhanced olfactory senses) since most of the Resistance units have dogs in the base.
Since the only known T-5000 unit so far was only briefly featured in Terminator Genisys, its combat abilities are largely unknown, however, it seemingly easily overpowered John Connor's elite squad of soldiers without sustaining any serious damage.