Lothario is a warrior's name that became a byword for seducers.
The name is Germanic. Etymonline traces the Italian given name Lothario to Old High German Hlothari or Hludher. The same source gave German Luther and French Lothaire, and Old English had its own form, Hloðhere. It meant "famous warrior." The pieces are lut, linked to "loud," and heri, "host, army," linked to "harry."
The change came in 1703, when Nicholas Rowe staged The Fair Penitent, a tragedy. Wikipedia says Lothario is a libertine who seduces and betrays Calista. Merriam-Webster says the name soon began to be used metaphorically and became a lasting label for a man of questionable morals.
Rowe did not invent the name for fiction. Wikipedia notes that William Davenant used it in The Cruel Brother in 1630. Wikipedia adds that Cervantes had a Lothario in Don Quixote in 1605, in the tale of the ill-advised curiosity. Rowe's play itself adapted The Fatal Dowry (1632) by Philip Massinger and Nathan Field.
English then started using it for any man like that. Etymonline records the common noun from 1756. Wikipedia places that first modern use in The World, No. 202. Merriam-Webster quotes Elizabeth Bonhote's The Parental Monitor (1796), which refers to the "gay Lotharios of the age."
Other writers kept the type alive. Wikipedia lists Charles Beckingham's 1728 poem "Sarah, the Quaker, to Lothario," about a rakish ex-priest. It says Samuel Richardson drew on the type for Robert Lovelace in Clarissa (1748).
Later writers used the name allusively. Wikipedia lists Edward Bulwer-Lytton in The Caxtons (1849) and Anthony Trollope in Barchester Towers (1857). It adds that the name is sometimes not capitalized because of this allusive use.
Notice the irony. Nothing in the meaning of the name suggests seduction. A "famous warrior" became a byword for seducers because of one stage character.