Agatha is a member of the Elite Four, and the oldest one to date. As such, she serves as an inspiration for senior Pokémon Trainers all around the region. She uses Ghost-type Pokémon, and other Pokémon with horrifying appearances.
As the quote above says, Agatha uses Ghost-type Pokemon, along with Pokemon with horrifying appearances. In Red, Blue, and Yellow, the only members of Agatha's team that aren't Ghost-type are Arbok and Golbat.
Golbat is a bat that is over five feet tall. Most of its body is its huge mouth. Also, as shown in its Red and Blue sprite, it has a giant tongue. I'm not sure about everyone else, but that seems pretty horrifying to me.
Arbok doesn't look quite as scary as Golbat does, but it's an eleven and a half foot tall snake. On top of that, it has what is supposed to look like a big face on its hood. Doesn't seem as scary as Golbat, but it definitely would be scary to see a snake that huge.
Also, Gastly, Haunter, and Gengar were the only Ghost-type Pokemon in Gen 1, and it would be pretty boring to have an important character just have a bunch of Haunters and Gengars.