Which type of tumor is known to cause a “bump” or mass under the skin in dogs?

The type of tumor that is most commonly associated with the development of a "bump" or mass under the skin in dogs is a lipoma. Lipomas are benign tumors that consist of adipose (fat) tissue and are typically soft, movable, and painless when palpated. These tumors are quite common in older dogs and may appear anywhere on the body, particularly in areas with more fatty tissue.

In contrast, while carcinomas, adenomas, and sarcomas can also form masses, they typically have different characteristics. Carcinomas are malignant tumors of epithelial origin and can present as masses, but they may also be accompanied by other clinical signs depending on the affected organ system. Adenomas are benign tumors arising from glandular tissue, which can sometimes form lumps but are less frequently presented as subcutaneous masses in dogs compared to lipomas. Sarcomas, which are malignant tumors of connective tissue, can create firm masses that may be fixed to underlying tissue, indicating their invasive nature.

Thus, the presence of a soft, well-defined bump under the skin in dogs is most characteristic of a lipoma. This distinguishes lipomas from other types of tumors that may not exhibit the same benign and mobile features.