Eddie has his Spot, and Grandpa has his Igor, the bat that hangs out in Grandpa's lab. He is sometimes a nuisance, but not always. When Grandpa was hiding out in the attic, Igor was right there, by his side. Herman has upset Igor a few times as well, causing him to run away. Herman is king at making the Munsters' pets run away. During one of these instances, Eddie needed him to take to the pet fair. Grandpa volunteered and turned into a bat to replace Igor. Unfortunately, Eddie traded "Igor" for a squirrel at the fair! "Igor" aka Grandpa, was almost launched into outer space.
Igor was originally a little bat, but later on in the series, he was much bigger. His only voice is his squeak.