The most difficult mathematics is that which you do not know. A surprising amount of mathematics is actually easy once you've learned it. Of course, once you learn the easy stuff, then you have to start tacking the deep stuff, and that gets harder.
An isosceles triangle has two sides of equal length. An isosceles triangle also has two angles of the same measure; namely, the angles opposite to the two sides of the same length; this fact is the content of the isosceles triangle theorem, which was known by Euclid.
An isosceles triangle is a triangle with (at least) two equal sides. [See source.] It is the equal sides characteristic that defines the isosceles triangle, not two equal angles. True, we can show that, if there are two equal sides, there must also be two equal angles. But the definition is about the sides, not the angles.