I personally theorize that Quintessons look so bizarre becuase they are in fact of some type of cybernetic lifeform. the egg like body we see is just a shell and on the inside you have the shriveled fetus like remains or possibly the brain and organs from when they used to be organic similar to a dalek why exactly they chose such body types? Maybe because it was similar to their original body? They for some reason prefer their multifaced tentacle forms or that design was simply most practical rather than have lots of delicate squishy organs sloshing around in a giant container with arms and legs. As for transformers I think it's just because a basic bipedal body type is the most practical. but even then it's not necesarrily the norm. Lots of transformers either have or seem to prefer spending most of their time in beast mode. Sky Lynx Trypticon and most of the cassette bots have animal modes over robot modes. The dinobots and allot of other animal/monster alt mode transformers seem to prefer being in those forms over robot. Plus just look at the movies (I'd use other sources but I think their pretty vague outside of the bay films). You see lots of robots especially decepticons with avian or insect like features some look almost like demons or fairytale monsters in steel plus you see lots of things like reverse joint knees bird like feet extra thumbs on hands or fingers set up in some odd fashion plus one or two that have an extra set of arms and whatever the heck Demolisher is with his crazy cart wheeling unicycle body.