The bestiary for the Ancient Lands is taking shape nicely. Selecting the wildlife and monsters for a setting is a part of worldbuilding I find particularly interesting, but doesn’t seem to be given much attention most of the time. There seems to be a common tendency to throw in pretty much every beast and critter the creators find interesting, but personally I think that’s something that doesn’t really work well. I’ve been reading through the old AD&D monster manuals again some time ago, and those who always surprise me the most are the Forgotten Realms appendices. Those are meant to cover creatures specifc to the setting that are not covered by the regular monster books. However most of them ended up completely forgotten and never mentioned again in other books, box sets, and 3rd edition. It’s not enough to simply write up a creature, it also needs to be woven into the rest of the setting and become part of it.
Take for example Dark Sun, which has the kang, mekillot, and inix, which barely resemble any animals found on earth and have no special abilities. But they are memorable because they have a very important role. They are the horses and camels of the setting, which are used by everyone who is sane enough to not try crossing the desert on foot. Eberron has such unique creatures as the quori and the warforged, which could easily be dismissed as silly ideas, but are among the best known features of the setting because they play an important role in the world. Dinosaurs are implied to be existing in some remote regions in almost all D&D settings, but only in Eberron is their presence really acknowledged. By having a race of deinonychus riding halfling barbarians!
Quality goes over quantity, and I vastly prefer the approach of not adding anything to a setting unless it is relevant in some way.