Pete adopted the stage name "Megan Thee Stallion" after she was called a "stallion" during her adolescence due to her height (5 ft 10 in (178 cm))[7] and "thicc" body frame;[10] voluptuous, statuesque women in the southern United States are sometimes colloquially called "stallions".[1]
That's actually shorter than I expected, IDK why I always expect celebs to be taller than they are in person.
Also the version I saw her give was that she decided on it after some grampa on a stoop type guy started jokingly cat calling her and saying she was "hot" like a stallion, I think it was meant more to be a joke about her being a big personality.