What is the hard lump on my dog’s neck?

A hard lump on the dog’s neck may be a mosquito bite that usually fades on its own in a few days. If the dog was just vaccinated, the lump may be from incompletely absorbed medication; apply a warm towel to improve circulation and absorption. Finally, the lump could be a skin inflammatory growth or subcutaneous tumor; take the dog to a veterinary hospital for a puncture biopsy to confirm the cause before treatment.