Despite popular belief, it is not the length of fiber that determines how well it will spin into yarn, but the diameter (also referred to as "microns") of the individual hairs. The finer the fiber, the softer the yarn.
Undercoats of mammals are usually made up from very thin, sometimes near microscopic hairs. The lengths of these hairs may vary from very short (less than ¼") to very long (over 2") depending upon the individual animal. Regardless of the length, this fine hair will produce a very soft, luxurious yarn.
Do not discard the beautiful and plentiful outer coat. The hairs of the outer coat are smooth. This acts as a reflector for the sunlight and helps to keep your pet cool in warmer seasons. The outer coat is most often richer in color, the thickness adds strength and the reflective properties add luster to your yarn.
A mixture of both under and outer hairs can work together to create a very unique and beautiful keepsake yarn.