XiaoXiong,
Good observation about monkey and chicken being little systems with lots of depth. I think, in truth, all the XY animals are "little systems of their own" - I think it just depends how much particular teachers in different lineages specialised in, learned and/or wanted to pass on.
For example, look at this guy's vids - it hints at the incredible depth and variety to be found in all the animals:
Snake
http://jp.youtube.com/watch?v=r_R-mqv-R2g&eSwallow
http://jp.youtube.com/watch?v=usjvB0XXG ... re=relatedHe has loads of vids of the 10 animals. In each one he practices a single method up and down the training area, stops then does the next. Now, if you linked these single movements belonging to each animal together you'd get something like your more detailed snake and monkey links. (In fact - taking out the characteristic Moslem XinYi interludes (e.g. the pause with the leg raised) its very similar it is to our Hebei XingYi animal links despite six to eight generations of separation.)
For many years it was only "XingYi" styles of the XinYi/XingYi family tree that were openly taught to the public. In many "XingYi" styles these animal links are limited to just one example movement, thus reducing the complexity of a rich and detailed system into something that could be more manageably taught to the general public (along with the 5 fists). It's no co-incidence that openly teaching stuff to the public (in a commercial way for the first time) coincided with simplification. As class sizes go up massively you can't keep the same level of details - I'm sure anybody here who has tried to teach people martial arts will appreciate that point.
How many animal "shapes" do you need to get a handle on an animal? That point is debatable - obviously many famous teachers have thought that one was sufficient - others have felt that more than one was helpful. Either way, it's always worth remembering that all animal shapes are just "fingers pointing to the moon" - the moon you should be looking for is the way the animal itself fights/hunts.
You find odd little things like "in our style we have 5 variations of horse" popping up here from now and again - I believe this is where it all comes from. Ultimately, of course, there is only one version of horse - and it lives in a field and eats hay