Gadfly (noun)

1. A person who persistently annoys or provokes others with criticism, schemes, ideas, demands, or requests.
2. An insect, especially a horsefly, that bites or annoys domestic animals.

Origin:

Mid 16th century: from obsolete gad "wanderer" + fly1.

Examples:

1. He was considered a gadfly by the political establishment.
2. She was a gadfly to the company's management.
3. The gadfly was a constant annoyance to the cattle.
4. He was known as a gadfly for his constant questioning of authority.
5. The presence of the gadfly made it difficult for the horse to graze peacefully.