I was looking for this and it bothered me how basically all concepts for chairs for beings with tails are either stools or normal chairs with a hole you're supposed to back into, so I made my own concept for Folicoatls.
In fairness the concepts I've seen are prolly intended for cats and dogs and the like, who have lithe and well mannered tails, but it ignores beings with meaty clumsy tails, and that's what I hope to serve with these! I'm still thinking if there's some other ways to improve the chair's stability though, cuz that hanging bit would be a potential breaking point, tell me what you think!
I am holding back on making a Chair Post today, so hard, but yeah this seems like a fair, quick solution to the problem! I told you elsewhere, but a swinging carabiner-like support would be good, too; something that balances the back rest so both sides can flex at about the same rate, instead of the whole back leaning to one side. for folis, I doubt you'd have this problem often, since their backs don't look like they'd really lean 'em back that much to begin with.
I have to think about this problem kinda often, because people with tails (or just with fully chair-incompatible body plans) are the norm in my setting.
If there's only one armrest, lefties and righties are going to favor different ones, and there are going to be situations where different sides of entry may be preferable. For example, it may be desirable in a restaurant to have the tail slot always open to the outside of the table to minimize awkward shuffling, where it might not be needed at a circular table.
Much to consider!
I propose a chair with a swinging stem. This is inspired by those old hand cranks, you could do something similar by putting the stem on rails but I suspect it would be a bit less sturdy and more liable to be shaky. If it was really sturdy the arms could be attached to the back, if not it would either need to be armless or have some sort of arms that could retract/fold back down into the seat.

iv basically built my career on building furniture for bug people (whose abdomens are basically a stiff unwieldy tail) and after a lot of careful research and experimentation, i can confidently say: they'd probably have all of these options. there'd be cheaper chairs with just holes to stuff your tail into, sturdier chairs with the side cut out (popular in workplaces), and really fancy oscillating fuck chairs for rich people and weirdos