In Tomb Raider 3 for instance, in India you will meet these small monkeys. They are harmless in the first level (Jungle), as long as you don't hurt any of them. One of the first monkeys encountered actually helps Lara as it steals a medipack from Lara's sight forcing her to chace it. The monkey leads Lara to the first lever hidden behind jungle plants.
From the second level (Temple Ruins) on they are aggressive and you have to kill them.