I do not know the etymology of the word "Sulo" (Суло) at this moment, but can say that if used as toponym for a small settlement, e.g. village, then it most probably coming from 'someones' personal name masculine form "Sulov" (Сулов) and transformed to neutral form "Sulovo" (Сулово) to fit the grammatical gender. That is for example "village Sulovo" (село Сулово).
Analogically, the same surname for the female person will be in feminine form "Sulova" (Сулова).