Altitudes of equilateral triangles. They show up a lot. They are regular polygons and can therefore also be referred to as regular triangles.
Now lets think about it the other way around.
In geometry an equilateral triangle is a triangle in which all three sides are equal. That is all three internal angles are also congruent to each other and are each 60. They show up a lot. Featured on Meta Opt-in alpha test for a new Stacks editor.