La magneto is rare among -o terminal words in that it is feminine (at least in some dialects). The DLE gives its ultimate etymology as:
Del lat. magnes, -ētis 'imán1'.
Given the word has clearly not been directly inherited from Latin1 2, what is its etymology? Does it explain why it is feminine?
1. The only occurrence of magneto before 1881 in the RAE's CORDE is one example from the 14th century: "... turniano magneto leboreto..." (Gran crónica de España, Juan Fernández de Heredia (1385)).
2. The only pre-19th century occurrences of it in Google ngrams are references to Ancient Magnetum (Magneto in Portuguese).