Sounds like you're talking about a carbon filter in the air intake. I haven't had the stock air intake on my car in 13 years so I don't really remember if it has one or not. If there is a carbon filter there, it's there for emission reasons. It's there to absorb gas fumes flowing back out through the intake when you turn off the car. Carbon filters aren't replaceable and don't need to be replaced. And they don't cause much air restriction to reduce MPG or HP.
Chrisdesil is on target, the device is a Vacuum Switching Valve (VSV). It also collects fumes from the EVAP system to prevent release into the atmosphere. No need to replace unless it is defective. But be sure the rubber hoses are attached tight and are not hard or brittle.