Herp, noun: A cold-blooded tetrapod. AKA herptile or herpetofauna.
A herpetologist is a zoologist who specializes in reptiles and amphibians. Amphibians belong in the class Amphibia, which includes all of the amphibians and nothing else. Reptiles belong in the class Reptilia, which is a godawful monstrosity. Technically, mammals and birds and reptiles are all part of the clade of Amniota, and the class of Reptilia should be retired; biologically, mice and robins are just reptiles that gave up on being cold-blooded. Moreover, amphibians shouldn't be anywhere in the mix until we move up to the superclass of Tetrapoda, which includes all vertebrates with four legs, and those descended from them (e.g., snakes and whales).
But... we still have herpetologists. No one was willing to break the news to them, so they still think that "Reptilia" is a thing, and that it is almost the same thing as Amphibia, but with more scales. So, if we are not willing to shape language to fit reality, we may as well change language to justify our preconceptions, and thus, the herp: that which a herpetologist studies.
Herps, and indeed reptiles, are a human-selected patchwork of animals that pick out some relatives that look the same and leave out others that look different, regardless of how closely related these various animals may be from a genetic standpoint. This sort of human-invented category is called a paraphyletic group, or a grade. Other popular paraphyletic groupings include the fish, with many fish more closely related to humans and T. Rexes than to each other, and trees, which should properly include most plants, if it is to include both palm and pine trees in the same group.