This is a question related to a very specific situation : I had an idea about an unorthodox build for an ape animal companion. Animals only have melee attacks by default, and even the rules mention that teaching an intelligent primate to use crafted weapons may not incline it to use them.
So the main element of my build idea is the use of improvised weapons, since hitting something hard with a big stick is way easier than wielding a sword, and throwing a rock is way more intuitive than archery.
There is one thing that got my attention : something as basic as a club (a non-masterwork club is just a piece of wood worth 0 gp) is considered a simple weapon. For most classes, this isn't a problem, but what about an ape that learnt to use improvised weapons ? Does it take the -4 non-proficiency penalty when using a simple branch as a club, or can an exception be made ?