I'm pretty sure the chapter 5 animals are not necessary for 100% completion achievement (even the guidebook doesn't go into detail about the animals in chapter 5), though are for every compendium box to be filled (I think).
It is virtually impossible, though, given the constraints on chp 5 to study, track and skin every available 'extra' animal - there's a fair few (not at all listed in official guide), some of which only seem to spawn in a single location at random and where you are guaranteed to die trying to get to them. Some folk on the to internet have spent 5+ hours trying to get them all and even then abandoned it as too hard and grindy.