Why is the dog’s butt so red?

The dog’s butt is red because of constipation

Dry stool will make it difficult for the dog to defecate, causing redness and swelling of the anus. You can use warm water or 2% sodium bicarbonate solution or liquid paraffin enema, and take 1 tablespoon of milk of magnesia orally.
When constipation is severe, surgery may be required to remove stool. To prevent constipation in dogs, you can feed them dog food that is high in fiber, low in oil and salt, and you can also give them probiotics to regulate their gastrointestinal tract.