My dog has a lump on his ear flap. What could it be and is it serious?

Hi and thanks for using Petco Pet Education Center, formerly Petcoach! Sorry to hear about Bertie. Lumps generally fall into a few broad categories including growths/tumors which can be benign (not cancer) or malignant (cancer), granulomas which are a collection of inflammatory cells, abscess/infection, among others. In young dogs, a common benign tumor called a histiocytoma can develop. These are raised growths, often pink and somewhat flat like a button. These growths oftentimes resolve on their own without treatment. I would advise that your veterinarian take a look at Bertie. They can get a detailed history and do a complete physical exam focusing on the area of the growth, They may advise diagnostics such as a fine needle aspirate or a biopsy of the growth to determine what it is. If they suspect a histiocytoma they may just advise monitoring the area but to also be on the lookout for any scratching of the ear/growth. Once a diagnosis is obtained or suspected, a sound treatment plan can be discussed. The ear is a tricky area for growths and their removal because of the lack of skin to close over the incision if they are removed. Thus, a growth in this area warrants a quick analysis by your vet. Good luck with Bertie.