I'm designing a creature which possesses a decently unusual trait: despite having a developed head, it's mouthparts are attached to a tentacle that retracts into the creature's belly when not in use, similar to a flatworm, but with a more elaborate structure and complex mouthparts at the end of it. Based on this, Could a creature evolve to have a mouth separated from it's head?
If you have a body plan without a head, you can put the digestive apparatus where it makes the most sense.
A problem is how "head" is defined for us - we think of heads like the familiar tetrapods have. A system with a feeding orifice separate from the "head" makes more sense as you get farther away from a body plan with a conventional head. We are used to having a lot of important stuff all piled into the head but there are successful body plans from different phyla that do it differently. Starfish have sensory organs on the tentacles and a more distributed means of locomotion (tiny feet), so it does not seem so unusual that their central "pharynx" (really an evertable stomach) is in another different place.
Moving digestion away from sensation, locomotion etc makes sense. But I struggle to think of a system where the digestive apparatus is decentralized - for example entirely in one (or more) appendages. Probably because digestion is so central to a creature that lives this way, and also resource intensive to maintain.