Some have recommended that I turn to previous existing data bases in order to create my own - and I must admit I dislike this idea. To begin with, I have no idea what sort of actual research has gone into these data bases. Secondly, they are no doubt non-geographically specific, which I already stated I need to have. Finally, I doubt very much that the behaviour of the animal in included - and at any rate, I need behaviours that are tailored for my world. For all these reasons, someone else's data base for someone else's world simply isn't going to work.
Listen close, my brethren - don't let others do your work for you. You rob yourself of the benefit of exploring the world for yourself. You steal from yourself all the skills you'll develop, the learning you'll gain - and most of all, the inspiration that will be yours from having read the materials for yourself! Have you learned nothing from university? We do not innovate and invent by depending upon the possibly lacklustre, lazy work of others. We gain genius through doing the work ourselves and doing it well.
With these habits in mind - good habits, gathered with wisdom - I took up a region of the earth where a relatively small jungle occurs: the Western Ghats of India. These are a string of mountains down the west side of the Indian Subcontinent, 2000 to 4000 feet high, extending from near the equator some 1200 miles towards the north. And from within that area, I obtained the following list of large animals. I'm afraid I skipped birds (for the most part) and fish, partly because those are both huge databases and at the same time not especially useful for what I have in mind. However, I know I will go back and do some reading about both, and perhaps see if inspiration hits. Aha.
Here's the list of real animals I found:
Now, let's compare that with the list from the good ol' DMG:
Still, this leaves a few original creatures. The black bear is clearly the sloth bear. We can replace the baboon with the macaque. The DMG should have included the wild boar and giant boar on their list, but we're here to fix that. We have the muntjac, the gaur and the sambar for herd animals - though the Monster Manual does provide figures for cattle and the stag.
The only purely magical creatures AD&D offers are the lamia and the weretiger. But I think we should be able to do better. The trick isn't to come up with a way to use the smaller animals, but to imagine better, more profound versions of the smaller animals - that's the way to expand the creatures that inhabit our little jungle.
The strangest is certainly the purple frog, also the pig-nosed frog. It's far too small to be anything on its own, but the details about it being bloated, with oral suckers, and making the sound of a chicken clucking must suggest something. We might imagine it six feet in diameter, preferring to live in shallow pools, being able to flatten itself or puff itself up to twice its ordinary size (bloating) - it could thereafter spit water perhaps mixed with slime, that transfers a disease, a sort of mild acid; the clucking might be a form of mind control (drawing the victim towards the oral suckers). The creature might be intelligent or not - though given its extreme ugliness, I like the idea of a superbrain controlling other creatures, perhaps clucking them into attacking. Alternately, the purple frog might be a gentle yogi-like creature, ancient and with wisdom, rare and sought out by players seeking knowledge. The field is wide open.
The civet is prized for its glands, that are used to make perfume - and while some might imagine making the small creature into something that steals food, I prefer a more elaborate, larger animal that uses it's perfume to transmit a pheromone - which may alter the party's perception of reality, produce slavery, produce odd lovemaking incentives, drive away interlopers or lure them to their deaths.
An intelligent form of the hornbill - ordinarily used in tribal ceremonies - might be in command over a village, directing the human tribe towards acts of evil, war, butchery or even kindness and gentle adoration. This is India, after all.
I'm only riffing. A proper action would be to take each creature and create at least one derivation from the original. A giant, non-poisonous snake that at least causes considerable damage. Bats that are independently weak and easily killed, that in large numbers produce a wind so intense that it scatters tents and animals - or, in swirling around a party member, potentially suffocates. The deer that exists as a familiar for a local swami. The langur that, from the trees, throws poisonous feces.
Steadily, we create a unique, independent jungle from the base materials - and the more we learn about the jungle itself, it's nature and it's singular effects upon its inhabitants, the better equipped we are to decide which sorts of profound, elaborate creatures ought to dwell there. Better than riffing, coming up with things off the tops of our heads, are ideas which suggest themselves from the source material.
I haven't read near enough on the Western Ghats to properly decide what monsters ought to dwell there - and in the process, I improve myself, too.