What stereotype was used by the Hutu against the Tutsi?

Rwanda used to be a German colony and the Germans realised there were two distinct tribes in the region (there is a third, but it makes up only 1% of the population). By supporting a Tutsi monarchy, they found a way to remain powerful in the region without devoting too many resources to it.

When Belgium was granted this colony after World War 1, they saw no reason to change the system, and allowed the Tutsis to become the class above the Hutus. The Hutus resented this, and when European presence left the region, the situation got worse. And the rest you probably know.