Meg is correct about the double coat of most dog breeds. My daughter has a Pom and she gives me the undercoat that she brushes out to spin.
Additionally, I have a couple of farm collies (a Border Collie & an English Shepherd). I collect the undercoat I brush from them to spin.
I prefer to use brushed undercoat instead of cut or shaved coat. Even though the dog's hair feels silky smooth, the outer coat generally spins up coarse.
Better start looking at drop spindles & spinning wheels!!