All Terrain Soup
All-Terrain-Walkers.
Star Wars's Imperial Military and the Grand Army it usurped really liked their acronyms (much like their real world counterparts). The most egeriogus examples being the assorted combat walkers, which all start with AT, an acronym for "All Terrain", which is supposedly one of the justifications for the legs in the first place.
AT-RT
AT-RTs On the table top
AT-PT
The oft forgotten (since it debuted in Dark Force Rising instead of the movies and didn't have a clear description till the Dark Force Rising Sourcebook) little cousin to the AT-ST. For the most part the role it once had in the story has been replaced with the AT-RT, since that fits the role of "small pre-Imperial walker" but actually appeared in the movies. The modern (3E onward) Sentinel looks like one with less armor.
AT-DP
All-Terrain-Defense-Pod. An early design for the AT-ST that was reused by Disney. No clear reason for it to exist was ever given besides Disney wanting to have a different looking AT-ST. Possibly to have a light walker that was more intimidating than the AT-RT, but not have letting AT-ST lose it's rep to being slapped around without too much effort.
In Rebels it's less than ideal, (it's only appearance so far) unable to gain even a temporary victory because they're the bad guys in a Disney cartoon, who aren't Thrawn.
AT-ST
"All Terrain-Scout Transport", aka "The Chicken Walker", aka "the thing that the Imperial Guard Sentinel is a shameless rip-off for". Basically a box-shaped control room built around a central heavy laser gun and mounted on two spindly legs.
The AT-ST on the Table-Top
Content
Content.
AT-AP
Artillery.
AT-TE
Spider Tank.
AT-AT
"All Terrain-Armored Transport". These are the big, BIG bastards amongst the All-Terrain Walkers; enormous quadrupedal armored transports who look almost like giant robot cows, but who aren't so funny when you realize they can shrug off heavy artillery like it was nothing. Slow as molasses, but they will get to where they're going, and the head-like cockpit's twin heavy lasers can dish out a fair bit of pain along the way.
AT-AT On IS the tabletop
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AT-M6
The First Order's Bigger Scarier Gorilla walker.
Gallery
First Order Variant Gallery
The First Order had a bunch of Fanboy variants of the walker's built.