This pup is 6 months old and this photo of the mouth shows retained
deciduous teeth (arrows). Deciduous teeth are the first set of teeth (baby teeth) which are normally replaced by the permanent or adult teeth between 3 and 6 months of age. In most dogs as the adult tooth comes in it pushes out the baby tooth to replace it (just like in children). However, in certain breeds, especially toy breeds such as toy poodles, Chihuahuas, Pomeranians and Yorkies, the adult teeth come in and the deciduous teeth are retained.

Food material can get caught in between the adult tooth and the deciduous
tooth leading to early onset of periodontal disease and loss of permanent
teeth. It is therefore recommended that these teeth be extracted. This
photo shows the same dog as above after the deciduous teeth have been
extracted. The mouth is sore for a few days and the dog is treated with
pain medications and soft food until healed.