When choosing yarn for a dog sweater there are a few important points to keep in mind. Wool is warm and is nice for small breeds that are particularly sensitive to the cold, while synthetic blends are very soft and inexpensive. Sock wool is a great choice for dog sweaters since it holds up well to many washings and keeps its shape. It is usually made up of a mixture of wool and polyacrylic. A sock yarn dog sweater is warm and robust which is a perfect combination.