Complex sentence: They were originally a small mixed-breed dog, often in height and it is thought that the original Shetland herding dogs were of Spitz type, and were crossed with collie-type sheepdogs from mainland Britain.
Split into simpler sentences: They were originally a small mixed-breed dog, often in height. It is thought that the original Shetland herding dogs were of Spitz type, and these were crossed with collie-type sheepdogs from mainland Britain.