Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian. I'd like to help with your concerns about Rimi and the red/brown structure that you found on his side.
Because this came about suddenly and we are in the right time of the year for them to be active, my first thought is that he has picked up a tick. They can look quite round when engorged with blood but if you look closely with a magnifying glass you should be able to see legs. Here are some pictures of ticks:
If you look closely and are sure that this is not a tick but actually is a skin mass
there are many possibilities given your description and I'll give them to you.
One possibility is a sebaceous adenoma. These are very benign growths of the sebaceous (oil) glands and can be cream colored to reddish or black depending upon how irritated they get. They are very common on the head, neck, trunk and legs/feet. Once one pops up they seem to multiply. But really it's just if the dog has the genetic predisposition to form one then they will continue to get them. I tend to remove them if they break open or bother the dog as then they can get infected. Or if they are in a bad spot. They aren't really warts and aren't caused by a virus so don't
respond to medication used to treat warts in people. These are more common in older dogs. Below is a picture of a sebaceous adenoma.
Dogs can get viral warts but they occur in young dogs and are generally on the
lips, mouth and sometimes the genital area. They would be very rare in this location and at his age.
Another likely cause is a benign growth called a histiocytoma. These growths pop up quickly, grow quickly for a month or so and then tend to regress and resolve on their own within 2 to 3 months. They are most common in young dogs on the face, ears and legs and paws. But older dogs can get them too and if they scratch them they may not resolve. If the dog rubs them or scratches they will hang around longer or can get infected. You can use a drop of cortisone cream on them to reduce inflammation if they aren't ruptured. If they are scratched open I would recommend an antibacterial ointment instead. If the bump doesn't resolve completely in 3 months it should be surgically removed as it is unlikely to go away on its own at that point and can lead to continual secondary infections. Below is a picture of a histiocytoma.
Another possibility is a tumor called a mast cell tumor. These have variable degrees of malignancy, some can be treated with just surgical removal, some are quite aggressive and will spread deep into tissue and to local lymph nodes. The only way to know for sure how malignant the tumor is is to remove it and have it biopsied. Below are pictures of mast cell tumors.
Other tumors that this may be include a basal cell tumor but these are much less common in dogs then in people or a melanoma. These tend to be fairly benign behaving in the skin and respond to surgical removal. A plasma cell tumor is another possibility.
Of these tumors none resolve completely on their own except a histiocytoma. Mast
cell tumors will change in size if they are bumped, and then go down slightly in size but never fully resolve on their own.
If you want to be absolutely certain of a diagnosis your veterinarian can perform a fine needle aspirate on the bump. He or she puts a tiny needle in the bump, draws cells out, and looks at the cells under the microscope to identify them.
I don't think this is an emergency but I do recommend having your veterinarian look at it and evaluate it for you.
Please let me know if you have any further questions.