This spelling was used by Lord Byron for characters in The Giaour (1813) and Don Juan (1819), and it is through him that the name was introduced to the English-speaking world.
Magenta was a character in “The Rocky Horror Picture Show” musical play and movie. She was a domestic maid played by Patricia Quinn.
Its variant Merryn was the name of a Cornish saint.
In the English-speaking world, all forms have been occasionally used from the 19th century onwards.