We know about embellishing 16th century pieces with divisions from sources like Ganassi and Ortiz.
According to Wiki the first reference to ornaments is in the late 1500s...
''Starting with Archilei (1589), the treatises bring in a new set of expressive devices called graces alongside the divisions. These have a lot more rhythmic interest and are filled with affect as composers took much more interest in text portrayal. It starts with the trillo and cascate, and by the time we reach Francesco Rognoni (1620) we are also told about fashionable ornaments: portar la voce, accento, tremolo, gruppo, esclamatione and intonatio.''
I would imagine that guitarists were doing this earlier than these writings. For me it sometimes feels right to enhance the rhythm in , say , some of the livelier pieces from Phalese.
Well there is, of course, Vincenzo Capirola (born in 1474) among the forerunners. He gave explanations translated into english and french in his book. What seems "unique" is his different playing (used in two pieces) on two strings belonging to the same course. I finally found my own way to play (rather quickly) this with the right hand : my teacher used index finger/index finger and I play "figueta extranjera"(thumb/index finger). I am not specialized in ornementation. I have a french "Baroque" book on the subject for any musical instrument (XVII- XVIII th Century). I have no opinion concerning "ornementation".