Before I knew the actual names for Scout Walkers or AT-STs, I simply knew them as "chicken walkers", as have many other Star Wars fans. However, I don't know where that term came from, I don't think the films or any official Star Wars media ever called it that, but I could be wrong. How did fans come to refer to AT-STs as "chicken walkers"? Where did that term come from?
Out of universe: The phrase 'chicken walkers' refer to the type of articulation in the legs. This term applies to all mechs with this type of articulation. Other mechs with this type of articulation: ED-209, AT-PT(features in the games), and the AT-RT (featured in RotS, and Clone Wars) This type of motion is often faster than a more 'man-type' motion, but supposedly can't handle rugged terrain as well. (Which I don't know if I agree with that idea, as we see it used almost exclusively in rugged terrain, but we do see an AT-ST slip on logs during the battle of endor)
In universe: There really is no reference to calling these things in the novels or movies.
Source: Wikipedia-Chicken Walker
Its legs bend in the same way a chicken's legs bend, and so when it walks, it looks like a chicken. It is worth noting that the AT-ST is not the only "chicken-walker", it is a more general term used across a variety of sci-fi stories to describe many different types of mech / robots with similar legs. The ED-209 in Robocop was also called a chicken walker, as was the X-1 Alpha in Future cop and many others. It is not a term specific to Star Wars.