Based on looks and Gru's bizarre collection, Kyle must be a mythical dog Chupacabra. Bluish, spiky sharp spines, few threads of fur, very pronounce fangs something that crosses a dragon with canine. Please take note that a Chupacabra legends denotes that when disturbed it leaves a leave a sulfur stench, this explains Kyle's effect on plants when it pees.
Gru simplified it as of a "dog" but actually confuses him too, Kyle was not identified in DM 2 because Lucy's data base presumably identifies only animals with known existing DNA thus a mythical undocumented species like a cryptid chupacabra fit Kyle to a T is not on the list
