Which of the following is a common type of equine parasite?

Strongyles are a common type of equine parasite, specifically categorized as large and small strongyles, which are part of the strongylid family. They are significant because they can cause severe health issues in horses, such as colic, weight loss, anemia, and overall poor health. These parasites inhabit the horse's intestines and can also migrate through the body, leading to more systemic problems.

While roundworms can also be a common parasite in young horses and may cause serious issues, strongyles are particularly notorious for their life cycle and potential to create severe damage. The other choices, such as ticks and flies, are external parasites that can affect horses, but they do not fall under the same categorization as intestinal parasites like strongyles. Thus, options involving external pests are not classified as equine parasites in the same context as strongyles.