Mortadella is a traditional Italian cured meat made primarily from pork. It often contains pistachios, peppercorns, and fat, but no horse meat or donkey meat is used.
The rumor about horse or donkey meat being used in mortadella is a myth. It's likely a misconception stemming from the fact that some cultures do utilize those meats in various dishes, and perhaps a misunderstanding of the ingredients in mortadella.
However, there is no evidence to support the claim that horse or donkey meat is used in the traditional production of mortadella.