This should be an easy one.
This is a boxer puppy with a cleft lip. It also appears to have a cleft palate., also known as a "harelip." The two malformations often occur together, and both are associated with a natural seem on the body not closing fully during development. Normally, it will be fully closed by birth. If you feel your own upper lip, the dip below your nose is that seem. Cleft lip and cleft palate are not very common occurrences in dogs and can occur in any breed. However, purebreds and the brachycephalic breeds especially are more prone to them. They are often repaired as can be seen here, though it depends on the severity. Cleft palates often will cause difficulty when the dogs eats or drinks because there is an opening connecting the mouth and nasal cavity, which can lead to infection. Puppies with cleft palates will often be slow developers because of the difficulties associated with the deformity.
Here is the same dog at a year old. Appearance of a cleft lip will often change with age.
Images are from Wikimedia Commons under a creative commons license: one, two