The Tavern

I want to suggest adding a new enemy. It should have two forms.
First form is ground. In it it resembles the good old goliath beetle - slow but tencious and well protected (and not only from physical damage).
If player moves a long distance from the beetle, it goes to the second form - flying. In it it loses protection, acquiring instead a high speed. Although he still has a lot of health, in this form he can be killed from the distance battle. Of course, if it don't get to the player before.
I think that such an enemy may well fit into the bestiary of the lair, playing the role of both a pursuer for the ranged characters, and obstacles for the melee characters.

if you can still eventually kill it unskilled with a +0 spear without any damage taken it's a bad monster, and honestly it sounds even more obnoxious than goliath beetles were because you're now required to do so to prevent it from charging at you

Well, probably it is because my bad english. Or my concept is really bad. I'll try to explain.
In first form it will have much hp, defence and damage, so killing him should not be easy.
The second form he will take in the main when chasing, if you move away from him for a sufficient distance. He will still keep his HP in it and will return to the ground form next to the player. So killing him unskilled with a spear +0 should be an impossible task, but the loss of protection should give long-range characters a chance to quickly destroy it in this form.

If it moves more slowly than the player and doesn't regen quickly, you can kill it with a +0 spear. If it does regen quickly you can walk away (and since it becomes fast when you are not close to it, you can lure it up stairs and park it without killing it. )

I thought about this a little and I came up with two solutions.
First is reflection ability but it does not solve the problem with the ease of avoiding a fight.
Second is short distant attack with acid or fire. Something like a bombardier beetle.

Thanks for the comments, anyway.

Isn't it supposed to have enough AC to prevent spear-kiting?

Well, what if you have a slightly better spear or a trident? Still means you just kite it a lot.

Just make it normal speed when moving on ground.

Why do people want slow monsters anyway?

AC can roll "0" and block no damage, so a monster with 1000 AC can still be killed by a spear. It'd be extremely rare, but it's technically possible. Depending on what range the monster switches states, and if the switch required any time, it'd probably still fall victim to kiting.

While it's somewhat safer to spear kite a monster you keep at range 2 or 3, it's also just fine to do it from range 1-2, so it'd have to get the speed boost as soon as it was one tile away to prevent spear kiting. It may be possible to have a monster whose speed is 8, but the speed doubles if you are not directly next to it? No switching time, just automatically gets double energy if the player isn't adjacent. That way the monster would get close to you easily, but not hit very often in combat?

In the end though, that's basically just the "it covers ground quickly" mechanic we already have, for example on ugly things.

How would you feel about a beetle with something like base 8 speed, and move: 50%? This accomplishes everything from the proposal except for the losing/gaining armor, really. Flying doesn't make much of a difference in lair, unless the player happened to learn airstrike. I suppose a gimmick of increased armor when next to the player is possible, but I'm not sure it would really make much of a difference - it's faster than you and will move back pretty quickly.

I'm not sure why the beetle changes. How about ... graboids and assblasters?