Dogs get lumps and bumps in various places, and it is not always clear what the cause of the lump is. It is best to get a veterinarian to examine your dog and decide whether the bump needs attention. A white bump on the dog's ear flap is most likely to be an insect bite, a wart or skin tag, a cyst, or an aural hematoma.
According to the Merck Veterinary Manual the outside part of the ear is called the pinna. This area can be affected by a variety of skin conditions. It is rare that an issue affects only the ear skin and nowhere else on the dog. A veterinarian will usually need to make a complete physical examination to determine the cause of the issue.
There are some issues that could cause bumps on the inside of a dog's ear flap and not appear as a problem anywhere else, such as an insect bite. According to Merck, an insect bite causes small, hard, round bumps that can be pimple-like bumps in dogs' ears. Insect bites from mosquitoes, flies, and fleas aren't serious, and may only need medication to control itching or inflammation.
Most insects bites or parasites will only cause redness, swelling, itching, or blistering. Mites are so small and difficult to see that a veterinarian may have to take a skin sample to make a positive identification. Mites appear as tiny white specks. In cases where it's tough to figure out what the problem is, your veterinarian may request bloodwork.
Like us, dogs have oil glands in their skin that are called sebaceous glands. These glands produce an oil called sebum. This helps keep hair soft and shiny. Occasionally though, this oil can clog pores. When a dog develops a sebaceous cyst, it looks small, white, raised bumps. You may be able to feel them under the skin.
A sebaceous cyst can happen anywhere on a dog's body. It simply means that a hair follicle or skin pore gets blocked by something. Dirt, debris, scar tissue, or an infection can cause a blockage that creates pimple-like bumps in dogs ears or anywhere on their body. It is more noticeable on a dog's ears, simply because the hair is thinner on that part of their body.
Aural or Ear Hematoma
Believe it or not, your dog can develop what is called an aural or ear hematoma just through something like vigorous head shaking. Usually, there's some kind of trauma involved such as accidentally banging their head on a table. But these fluid-filled swellings can develop on a dog's ear flap.
Your dog's ears have many tiny blood vessels. Vigorous shaking or an accident or injury can cause many of these tiny blood vessels to burst. This fluid could appear as bumps on the inside of a dog's ear flap. Sometimes, also, ear scratching can cause the problem, as can environmental or food allergies that affect the ear canals. These can be painful for your dog, and your vet may need to drain the area, although hematomas do eventually disappear on their own. Sometimes, the swelling can leave the dog with a "cauliflower" ear from scarring.
Warts are another reason that you might see bumps on the inside of a dog's ear flap. Warts are actually caused by a virus called the papilloma virus. These viruses case small misshapen growths in skin or in a dog's mouth. Any dog can be affected by wart-causing viruses, but young dogs who have been exposed to the virus or immune-deficient dogs are at risk, as are dogs who simply grow skin lesions due to aging.