How did Pluto start being called the dwarf planet?

Answer 1

they decided to call it a dwarf planet because originaly it was called a planet, but then they had some kind of 'planet test' and unluckely pluto did not pass the size test, and was counted to small and was called the dwarf planet.

Answer 2

Pluto started being called a dwarf planet in 2006 when the International Astronomical Union (IAU) redefined the definition of a planet. According to the new definition, a planet must clear its orbit of other debris, and Pluto does not meet this criterion. Hence, it was classified as a dwarf planet instead.