A completely under utilized "monster" in most campaigns are real life
animals. The familiarity players have with these beasts
automatically lends a sense of "realism" to any setting. Players
know how most of these animals look, smell and sound like from
childhood (or in my case monthly) trips to the zoo. The zoo is a
great place for inspiration. I have seen spider monkeys with visitor
guides that have fallen in to their exhibits.

That gave me the idea of a spider monkey that followed a party along
and would steal items while the slept and failed spot checks while on
watch. Once the party awoke to find the little deviant monkey
"reading" their map of the ruins they were in. That lead to a very
fun chase, setting off of traps as they forgot where they were, all
from a real life "monster".

Your heroes are used to slaying denizens of the nine hells, orc kings,
and other fantastical magical creations. But if you somehow have
their party dehydrated, on a savanna, surrounded by a pack of starved,
frenzied hienas they will have to get creative. Being stalked in a
jungle by a tiger, panther, or diseased orangutans can make for some
interesting gaming flavor. So visit the zoo with a camera and a
notebook for some simple yet effective "monster" encounter ideas!