Ah you have it backwards - there was already the Tutsi ethnic group, but the Belgians found it easier to identify them as Tutsi by number of cattle etc.
"Prior to the arrival of colonists, Rwanda had been ruled by a Tutsi-dominated monarchy since the 15th century."
"Rwanda was ruled as a colony by Germany (from 1897 to 1916) and by Belgium (from 1922 to 1961). Both the Tutsi and Hutu had been the traditional governing elite, but both colonial powers allowed only the Tutsi to be educated and to participate in the colonial government. Such discriminatory policies engendered resentment."
"When the Belgians took over, they believed it could be better governed if they continued to identify the different populations. In the 1920s, they required people to identify with a particular ethnic group and classified them accordingly in censuses."
"Prior to the arrival of colonists, Rwanda had been ruled by a Tutsi-dominated monarchy since the 15th century."
"Rwanda was ruled as a colony by Germany (from 1897 to 1916) and by Belgium (from 1922 to 1961). Both the Tutsi and Hutu had been the traditional governing elite, but both colonial powers allowed only the Tutsi to be educated and to participate in the colonial government. Such discriminatory policies engendered resentment."
"When the Belgians took over, they believed it could be better governed if they continued to identify the different populations. In the 1920s, they required people to identify with a particular ethnic group and classified them accordingly in censuses."