Skin tags, collagenous nevi and fibromas are lumps made mostly of fibrous tissue. In a lab report from a skin sample, a skin tag may be called an acrochordon or a fibroepithelial polyp, and a collagenous nevus may be called a collagenous hamartoma [1][3]. All three are usually benign, which means not cancer.
They can look like other lumps, including ones that are not benign. Researchers who reviewed thousands of dog and cat tumors stress that even a skin nodule that looks harmless needs a lab check to be sure [5].
Have your vet check any new lump. Never tie off or cut a lump at home.
Skin tags (acrochordons)
A skin tag is a small growth that sticks out from the skin, often on a narrow stalk. Under the microscope it has a core of fibrous tissue and small blood vessels, covered by a layer of skin that may be a little thickened [1][2].
- Who gets them: mostly adult and older dogs. In a study of 225 skin lumps from dogs in Grenada, fibroepithelial polyps were one of the most common lumps that were not tumors, and the dogs averaged about 7 years of age [3].
- Look: a raised or hanging lump on the skin surface. Some growths that look like this are warts instead; for those, see warts and papillomas in dogs and cats.
- More may follow: a dog can have several. In rare cases many skin tags grow close together and form a patch called an acrochordonous plaque. Swiss pathologists described this on the back of the neck in two Bulldogs and a Pug and suggested Bulldog-type breeds may be prone to it [1].
Collagenous nevi
A collagenous nevus (collagenous hamartoma) is a small, local overgrowth of collagen, the tough protein that gives skin its strength. It is not a tumor that spreads [3].
- How common: they turn up now and then in skin samples from dogs; in the Grenada study they made up a small share of the lumps that were not tumors [3].
- Age: the dogs in that study averaged between 7 and 8 years of age [3].
- Look: firm nodules in or just under the skin.
Some dogs, most often German Shepherds, get many firm collagen nodules, mostly on the legs and head. This condition is often linked to kidney disease [4]. For more, see nodular dermatofibrosis in German Shepherds.
Fibromas
A fibroma is a benign tumor of fibrous tissue. On the outside it can look much like a collagenous nevus or a skin tag, and a soft fibroma and a skin tag can look the same under the microscope too [2].
- Dogs: in a Lithuanian study of dog and cat tumors, fibromas made up about one in 18 canine tumors. They were found in purebred and mixed-breed dogs in roughly equal numbers [5]. About half of them were in the mouth rather than the skin [5].
- Cats: fibromas were clearly less common in cats than in dogs in the same study [5].
- Look: a single, firm or soft, raised lump in or under the skin. Only a lab test can confirm it.
How vets diagnose them
A vet can take cells with a fine needle (cytology) or remove a small piece or the whole lump for a lab exam (biopsy). A recent study of canine skin lumps found that a firm diagnosis still rests on testing the cells or tissue [6]. The lab result tells these lumps from others, including cancers that can look similar [5].
For other lumps, see lumps and bumps on dogs and lumps and bumps on cats.
Treatment
Your vet will advise on treatment for your pet. In general:
- Skin tags: many are simply watched. A vet may remove one that bothers the pet, bleeds or gets caught, and send it to the lab to confirm what it is [2].
- Collagenous nevi and fibromas: because they are benign, removal is not always needed. Your vet may suggest surgery if a lump grows, changes, gets in the way or the diagnosis is unclear.
- Many nodules: a dog with many firm collagen nodules may need checks of the kidneys as well as the skin [4].
At home, as directed by a vet:
- note the size, place and date of any lump, so you can show your vet any change;
- never tie off, cut, squeeze or try to drain a lump;
- go back for every recheck.
Outlook
Skin tags, collagenous nevi and fibromas are benign growths [1][2][3]. The main reason to test them is to make sure a lump is not something else [5].
These are statements about groups of animals. They cannot predict how one pet will do. Your vet can explain what this means for your pet.
When to see your vet
See your vet if your dog or cat has:
- any new lump on the skin;
- a growth on a stalk, or a patch of many small growths [1];
- a known lump that changes, grows, bleeds or bothers your pet.
FAQ
Can I remove a skin tag at home?
No. Never tie off or cut a lump at home. A skin tag can look like other lumps, so your vet needs to see it and may send it to the lab [5].
Are skin tags cancer?
Skin tags are benign [2]. Only your vet can tell a skin tag from other lumps, so have any new lump checked.
My dog has one skin tag. Will more appear?
They may. Some dogs have several, and a few grow many close together [1].
The bottom line
Skin tags, collagenous nevi and fibromas are benign lumps of fibrous tissue, seen mostly in adult and older dogs and less often in cats. Treatment is often optional, but they can look like other lumps, so have your vet check any new lump and never remove one at home.
This article is for general information only and does not replace advice from your veterinarian. If your pet has a new lump, see your vet.
Sources
- Bidaut AP, Gross TL, Noli C, Welle M, Suter MM. Acrochordonous plaques in two Bulldogs and a Pug dog. Veterinary Dermatology. 2003;14(3):177-179 (PubMed abstract). https://pubmed.ncbi.nlm.nih.gov/12791052/
- Foroughi-Gilvaee MR, et al. Synchronous Multiple Primary Tumors in a Dog: A Case Report. Case Reports in Veterinary Medicine. 2026 (open access, CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC13435256/
- Chikweto A, McNeil P, Bhaiyat MI, Stone D, Sharma RN. Neoplastic and nonneoplastic cutaneous tumors of dogs in Grenada, West Indies. ISRN Veterinary Science. 2011;2011:416435 (open access, CC BY 3.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC3658601/
- Gómez-Barris BR, Balazs V, Villamar KC, Toro-Valdivieso C. First case of nodular dermatofibrosis with cystic renal disease in a beagle. Veterinary Medicine and Science. 2024 (open access, CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC10918974/
- Juodžiukynienė N, et al. Retrospective Analysis of Prevalence of Tumors in Dogs and Cats in Lithuania. Veterinary Sciences. 2025 (open access, CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC12656924/
- Dank G, et al. Tumor-Specific Classification of Canine Cutaneous and Subcutaneous Tumors Using Heat-Diffusion Imaging and Artificial Intelligence. Veterinary Sciences. 2026 (open access, CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC13611836/