So, I made the slight mistake of worldbuilding by starting off with just a planet's local sentient species and working from there.
This species, the Trillek, have a more or less theropod appearance with some minor anatomical differences, the most jarring being a prehensile tail with two additional opposable appendages that can form a hand.
Initially, this was just going to be a trait unique to their immediate order, the appendages something like fleshy tentacles, but I've started bumping the trait even further back, potentially as bony limbs.
It made sense for the Trillek, needing high prehensility for their dense jungle origins. But I have a hard time making this feature make sense in other animals.
I drew up two basal lizard-form, one with a hand-tail and one with the appendages forming a new set of legs, making it a hexapod.
One useful form I made for the tail-appendages was also a pterosaur-like hexapedal configuration: they form landing legs in place of the main legs, which function as a massive soaring wing, and the forelegs potentially functioning as a flapping wing.
For the terrestrial herbivores, I'm afraid I've had a lapse in creativity. I've gone for standard quadrupedal herbivore shapes, but with more substantial tails that are good muscle attachments to the legs. The appendages on the larger animal I made has solidified into horn, and the appendages on the smaller one form part of a membrane that runs from the tail tip, to the ends of the tailbranch, to the base of the tail as a signalling mechanism.
Other than that, I have trouble figuring out why this part of anatomy would be kept beyond its basal function for grabbing a hold of branches or tunnel walls. Should I go back to having it be unique to one order? Or potentially just make everything a hexapod and the tail-branches vestigial rear limbs? Or are there ways I can actually make this work in a general sense?
More to the point, I'm trying to figure out if tail appendages are useful and versatile enough that they can appear in a wide variety of species, if not be a standard trait for all the planet's vertebrates.
Here are some sketches for the general concept:
Basal conceptuals. Top is a basic lizzurr with a grasping branchtail. Probably works okay for climbing. Wrestling competitors. Mating? Maybe. Doesn't like having his picture drawn.
Bottom: Hexagekko. Branch tail makes a primitive extra limb pair. Why? Well, maybe you just need some extra spring in your step. Or an anchor point.
Trillek, the dinosauroid boogers whose anatomy has accidentally thrown me into a loop.
Bottom right is the well-developed tail-hand. The feathers can open up to allow for the appendages to curl (up or down!) or close to form a display fan. Provocative.
Beastie Boys from the cave painting. Dewlap antelope's branchtail can move the attached membrane up, down, or fold shut to communicate. Could also work as a radiator?
Tricerobuffalant's tail branches, however, are covered in horn and immobile :(
WIP of a four-limbed flying creature. Front wings flap, rear wings soar. Might invert configuration. Wings fold to become limbs; tail-legs are structural support and landing gear.