It looks like an ordinary walnut. However, it can also have a nut translator on it.
Role in episode
After SpongeBob shows Gary's new collar, Sandy claims that she made a nut translator ages ago, and uses it on a walnut. The walnut then exclaims that they can't trust the pistachios, due to them working for the peanut mafia. Sandy then laughs it off, saying that walnuts are extra nutty.
- The walnut acts similarly to the peanut in the episode "Chimps Ahoy," as both are sentient nuts (though peanuts are legumes) that use a nut translator in order to be able to speak.