Big ol' stompy walking war crimes! Not in the least bit realistic, but man are they ever awesome! For the purposes of this post a mech is any limbed or otherwise zoomorphic vehicle that is controlled by a living pilot either internally or externally.
How did they come about?
What range of sizes do they come in? How do larger models get around the square-cube law (if you care to justify such things).
How are they piloted? Do they require a crew or can they be controlled solo?
What are their cultural associations? Are mech pilots highly esteemed or is the job just another job? Are mechs themselves worshiped as divine avatars? Are they merely tools of war?
Are their non-combat uses for mechs? Are they used in sports?
Are mechs commonplace or are they rare and powerful?
Can they transform?
I've already answered most of these questions for my own setting, but here's a rundown since Lemmy's default post sorting algorithm does not facilitate long-term discussions in the same way traditional forums do.
Yinrih went from conventional armor to powered armor, then gradually built bigger and bulkier until you weren't wearing it so much as piloting it.
What humans refer to as "mechs" are considered by yinrih to be two different classes of machine. A smaller class of externally piloted micro mechs are chiefly used in medicine, and can range from cell- to pill-sized. True mechs are piloted internally. There's a smooth size gradient going from powered armor to "mini mechs" that are too large to be considered mere armor but small enough to negotiate some indoor spaces. Mechs in the usual sense can be quite large, though not quite skyscraper-sized, mostly because yinrih are not bipedal and thus mechs are longer than they are tall.
They can, however, climb skyscrapers. Yinrih are arboreal, so their mechs are adept climbers too. They achieve this through force projector plates located on the palms and digits of the mech's paws. Force projectors generate a normal force when a voltage is applied across them. This force can be directed outward or inward, either sticking the plate to whatever it's touching or reducing the pressure on the surface the mech is standing on, preventing the mech from damaging the ground underfoot.
Mechs can be piloted solo, but usually have a crew of two.
Mechs are heavily associated with the Knights of the Sun, a Claravian order of warrior monks. These monks have many of the same cultural associations as European knights, with mechs replacing horses. The knight pilots the mech while a squire acts as an engineer and sometimes mans secondary weapons.
Internally piloted mechs don't see much use outside of combat, though they do have mech combat sports. Micro mechs on the other hand are ridiculously versatile. The Commonthroat word for micro mech literally means "avatar" or "representative", and you can infer from this that they're used in places where manual (and caudal) dexterity is required but that may be too dangerous to do in person.
The mechs used by the Knights are often named and treated as more than mere tools, but it's not quite at the same level as a super robot anime.
I haven't decided if there are transforming mechs yet, though there may be flight-capable mechs used on the floating cities of the gas giant Welkinstead.
Here's the finished version of the drawing I posted earlier. You gotta have caution stripes, it's the law. There's also now a bank of tail-actuated analog controls located at the base of the seat. That whole platform is actually the inside of a hatch located on the mech's belly.