Possibly from a personal name *Formosellus, also in genitive, which would be a variant with suffixation with respect to well attested anthroponyms such as Formosinus, Formosus, from the Latin formosus “beautiful”. See Riesco Chueca (539-540).

Cited bibliography: 

-Riesco Chueca, Pascual. Toponimia de la provincia de Zamora. Panorámica documental, comparativa y descriptiva. Zamora: Diputación de Zamora, Instituto de Estudios Zamoranos “Florián de Ocampo”, 2018.