What is a "mencho"?

My guess is that it's a nickname. (And Wikipedia thinks so too.) Many Spanish names have nicknames where an S sound is replaced by CH and some other simplifications occur, for example, Lencho for Lorenzo, Pancho for Francisco and Chucho for Jesús. Thus, Frumencio and Nemesio (or Nemecio) are associated with the nickname Mencho. Therefore, Nemesio Oseguera Cervantes, who is in the news right now, has the nickname “Mencho.” (See for example "Cómo obra la fonética infantil en la formación de los hipocorísticos" Author(s): Peter Boyd-Bowman Source: Nueva Revista de Filología Hispánica, Año 9, No. 4 (Oct. - Dec., 1955), pp. 337-366 Published by: El Colegio de Mexico Stable URL: https://www.jstor.org/stable/40296949 Accessed: 24-08-2019 23:29 UTC.)

In Spanish you can put the definite article in front of someone's first name or nickname to express familiarity. (See for example: link, which states, "este uso del artículo encierra afecto, cariño y familiaridad.")

Put those two elements together and we see that someone named Nemesio could become known as El Mencho.