There is some evidence of a certain species of bird that has some stuttering like behavior. However, based on what we know about stuttering I think you'd be hard pressed to find it in the animal kingdom. Stuttering is not a problem with speech alone but with speech and language. Our mouths and muscles and tongues work perfectly fine. We can make noises without stuttering, even make the sounds of letters (tatatata) or nonsense words (pataku) but the second our words start to carry meaning, ie language is introduced to the system, stuttering starts....So if it exists I would only expect to find it in higher intelligence animals, like chimps or dolphins.