Bats are animals that appear briefly in both Uncharted: Drake's Fortune and Uncharted 2: Among Thieves. They serve the same purpose in both games: briefly startling the main characters as they explore ancient ruins.
Uncharted: Drake's Fortune
In Chapter 2 - The Search for El Dorado, Nathan Drake and Victor Sullivan enter the temple that once housed El Dorado by smashing a rock through a false wooden section of ground in the jungle. As Nate enters the darkened temple, he turns on his flashlight. A group of bats fly at him, startling him.
Uncharted 2: Among Thieves
When Nathan Drake, Chloe Frazer and Victor Sullivan are exploring the Borneo caverns, they light up the room with their torches. When they enter a room filled with barrels, they startle a colony of bats which fly out, surprising the trio.