What's the deal with tennis balls?

We recently noticed Brownie's deep obsession with playing with his tennis balls. He has small and medium size balls which we play fetch with. He can paw at the ball and try forever to get his mouth around the medium size one evern though it doesn't fit. He is just so silly! His all time obsession is plush long toys in shapes of snakes, long horses, octapus, or anything similar. We have bought everything we have seen. If anyone has anything interesting plush toys to share, please let us know! Brownie loves them!