This one is the largest I've got so far, a 33m carnivore.
Having that stack snacking on my 500 man MG battalions in the early game was annoying.
But these herbivores from another game is the most dangerous alien stack I ever encountered, starting out with 8700 when I found them roaming a ruined megacity in the early game while I was still at the automatic rifle/padded environment suit stage.
When the stack stampeded in the direction of troops it flattened them without taking casualties, so I mostly tried just to stay out of the way. But I still needed to maintain my logistics connections between cities, which occasionally required leaving troops on roads and hoping it would go somewhere else.
The 8200 screenshot is after I finally inflicted 500 casualties on it when the Astrazogots decided that flattening a stack with an infantry/MG battalion, its HQ unit, and a GR infantry battalion was a good idea. It succeeded too. But at least it lost 5.7% of the beasts.