Simple Summary: Rodents have become increasingly popular as household pets, but few studies have focused on tumors in pet rodents. Pet rodents differ from their laboratory counterparts in many aspects such as geographical preference of pet species or breeds, household raising conditions, population density, and life expectancy. Therefore, it is worth conducting this pathological study of 77 spontaneous tumors from 70 pet rodents. Overall, mammary gland tumors were common. In the meantime, mouse mammary tumor virus (MMTV)-like sequences were found in the tissues of dogs, cats, and humans, raising the concern of whether these pets may incidentally serve as a vehicle for the cross-species transmission of MMTV to household personnel. For that purpose, nuclei acid extracted from the formalin-fixed paraffine-embedded tissues of 20 rodent mammary gland tumors was detected for MMTV env gene sequences. All tumors revealed negative results of MMTV detection. Compared to the number of studies on the neoplasms of laboratory rodents, fewer studies have focused on spontaneous neoplasms in pet rodents. Notably, the mouse mammary tumor virus (MMTV) is associated with mammary tumors in rodents. In this study, 77 tumors and tumor-like lesions of biopsy samples were collected from 70 pet rodents, including hamsters (n = 47), guinea pigs (n = 16), unknown species (n = 4), rats (n = 2), and a gerbil. Fifty tumors were collected from 47 hamsters, in which the most common tumors were mammary tumors (13/50), followed by fibrosarcoma (9/50), mast cell tumors (4/50), and squamous cell carcinoma (4/50). The collected subtypes of mammary tumors in hamsters included tubular carcinoma (n = 5), tubular adenoma (n = 4), carcinoma and malignant myoepithelioma (n = 1), simple tubular carcinoma (n = 1), adenosquamous carcinoma (n = 1), and tubulopapillary adenoma (n = 1). In addition, twenty tumors were collected from guinea pigs, in which the most common tumor was lipoma (6/20), followed by adenocarcinoma of the mammary gland (4/20), trichofolliculoma (2/20), and collagenous hamartomas (2/20). In guinea pigs, the subtypes of mammary gland tumors were tubular carcinoma (n = 2), tubular and solid carcinoma (n = 1), and tubulopapillary carcinoma (n = 1). In 20 cases of mammary tumors, MMTV was not detected, implicating no evidence of MMTV infection in mammary oncogenesis in pet rodents in Taiwan. [ABSTRACT FROM AUTHOR]