Consider centaurs, mermaids/mermen/merfolk, driders/arachne, lamia/naga, and other half-human creatures with similar bodyplans, which I will henceforth refer to as "centauromorphs". They all have a legless human body (which may or may not include the pelvic region) attached to an animal body where its head would normally be; this applies even to the snake-based lamia/naga, because anatomically speaking, a snake's body is mostly an extremely lengthened thorax, with the actual tail only forming a small portion at the end.
So, this begs the question: How would the anatomical terminology work for a centauromorph, considering that there would be duplicate body regions and parts between the humanlike and nonhumanlike halves of the whole body? This is further complicated by the fact that those regions/parts aren't always truly homologous (e.g. the thorax in humans and horses contains the respiratory system and the heart); they can be merely analogous (e.g. the abdomen in humans and insects; whereas it's mostly the digestive and urogenital systems in the former, it contains practically every major internal body system in the latter except the brain, including the lungs and heart that would be found in the thorax in vertebrate animals)?
Furthermore, in the case of arachnid- or crustacean-based centauromorphs, the animal body part that the human portion would be attached to is the cephalothorax (i.e. it's a head and a thorax in one in the animal that the hybrid creature is based on); I don't know about anyone else, but describing a drider as having a legless humanoid body attached at the lower end to the front of a spider's cephalothorax sounds really awkward to me unless it's meant literally, i.e. if you actually amputate a humanoid's legs (or just use a humanoid who lost their legs beforehand) and surgically/magically attach them by the pelvis to an already-existing spider's cephalothorax.
For an example of what seems IMO to be improper anatomical terminology for such duplicate body parts, take Monster Musume's Arachne race (WARNING: it's a fanservice-heavy manga/anime, thus expect frequent NSFW-ness of images on the site). In the official diagram for Arachne anatomy, the thorax and abdomen are qualified with the adjectives "first" (for the human ones) and "second" (for the spider ones). This seems quite lackluster, because anatomical terminology typically proceeds in a medial/proximal to lateral/distal direction with respect to the standard anatomical position, the only exception I know being the numerical order of the digits (it starts from the thumb, which in SAP is actually the most laterally positioned of the digits). Furthermore, it gives preconceptions about the roles of each body region (especially their internal makeup) that do not seem to make any sense biologically; why would a centauromorph have two thoraxes that are separated by an abdomen, with yet another abdomen coming after them all?
PS: This question is particularly relevant in the case of one particular fictional arthropod-based centauromorph species that I'm working on, since its brain is actually distributed between the human head and what would be an arachnid/crustacean cephalothorax (i.e. effectively two brains), but that's a topic for another time I'm afraid, for the simple reason that I'm planning to make a dedicated question for said species considering how much issues I would like to ask for help about.