Are Jehovah's Witnesses Protestants? Are Jehovah's Witnesses Protestants, or are their own separate category?
No,Jehovah’s Witnesses do not consider themselves a Protestant religion.
Merriam-Webster’s Collegiate Dictionary, 11th Edition, defines Protestant as

a member of any of several church denominations denying the universal authority of the Pope and affirming the Reformation principles of justification by faith alone, the priesthood of all believers, and the primacy of the Bible as the only source of revealed truth.”

It is certainly true that Jehovah’s Witnesses deny the universal authority of the pope and fully support the primacy of the Bible but they  differ from Protestant religions in many significant ways. In fact, The Encyclopedia of Religion refers to Jehovah’s Witnesses as being “distinctive.”
There is this brief explanation of 3 basic differences between Protestants and Jehovah’s witnesses on our official website JW.Org

First, although Protestant faiths reject certain features of Catholic worship, Reformation leaders retained certain Catholic dogmas, such as belief in the Trinity, hellfire, and the immortality of the human soul. Jehovah’s Witnesses, however, believe that those doctrines not only contradict the Bible but also promote a distorted view of God. —See pages 4-7 of this magazine.
Second, the religion that Jehovah’s Witnesses advocate is, not one of negative protest, but one of positive instruction. They take seriously the Bible’s counsel: “A servant of the Lord is not to engage in quarrels, but has to be kind to everyone, a good teacher, and patient. He has to be gentle when he corrects people who dispute what he says.” (2 Timothy 2:24, 25, The Jerusalem Bible) Jehovah’s Witnesses do point out contradictions between what the Bible says and what many religious groups teach. Yet, their goal in doing so is not to reform other religious organizations. Rather, their goal is to help sincere individuals to gain accurate knowledge of God and of his Word, the Bible. (Colossians 1:9, 10) When people of other persuasions insistently disagree with them, Jehovah’s Witnesses avoid engaging in fruitless debates. —2 Timothy 2:23.
Third, unlike the Protestant movement, which has splintered into of denominations, Jehovah’s Witnesses have maintained a united global brotherhood. When it comes to Bible doctrine, Jehovah’s Witnesses in over 230 countries follow the apostle Paul’s counsel to “speak in agreement.” There are no divisions among them. Instead, they are genuinely “united in the same mind and in the same line of thought.” (1 Corinthians 1:10) They strive within their own ranks “to observe the oneness of the spirit in the uniting bond of peace.” —Ephesians 4:3.