Jacob McAllen's creations, intended to serve him. They appear to be one of the Vanguard's many experimental programs. When April arrives in McAllen's office, she sees that he has in vitro tubes with mutated humans growing in them, and McAllen sets a mutant on April in the laboratory, but it isn't clear if these are the warrior Shifters he's referring to.
The implication is that they would serve McAllen in Arcadia, either carrying out his orders or gaining intelligence on his behalf. How he developed them is not known, but it would appear that McAllen somehow knows how to induce the ability to Shift. As it is never explained whether the Shifting talent is genetically inherited or conferred in some other way, this raises some disturbing questions. It is not known what happened to the warrior Shifters after McAllen's disappearance.