I believe it is quite normal. I am autistic and I have been acting like a dog since I was 2, I am currently 13. My girlfriend is the same but she is a cat. I don't think It's always autism related because I know ordinary people who do the same actions, its just imagination or a relaxation thing to stop them from being stressed. It certainly is for me.
My daughter used to act like an dog over a period of about 2 years. She would sleep on the floor, eat out of dishes on the floor, make animal noises, and play animal games. She is now 17 and perfectly normal. She remembers that pretend time as great fun because I used to act like nothing was abnormal about having a dog daughter. I think that a sense of imagination is a wonderful thing!
i have been around a ton of kids that do this
its one of the symptoms of aspergers, but dont fret, this does not mean your child has this condition
there are a ton of asper kids in my parent group and all of them make animal noises
google
aspergers and animal noises