I would call the dog a mixed breed dog. The thing is, without knowing what was used to create this particular dog, short of DNA analysis (which gives a rough approximation of breeds present) you are guessing genotype by evaluation of phenotype.
Generally, mixed breed dogs are the offspring of the pairing of two mixed breed dogs. Each breed that you see today was originally the product of selective breeding. If every Pug were destroyed today, you could come up with a Pug breed again by repeatedly selecting dogs with Pug like traits.
The dog in the picture may very well be a “X mix” where X is Lab, Chessie, etc. Or it could just as likely be a beagleish line crossed with a Portuguese water dogish line.