OK, OK, I do know some ex-Parelli peeps and can sort of answer this.
An extrovert is a horse that is outgoing, social (even with people) , questions you when you teach new things or just accepts things when you teach new things. They will actively let you know when they are pleased and displeased with training, handling, etc. An introvert is not particularly outgoing, not particularly social with other horses or humans, not crazy easy to teach new things to because they don’t particularly want to accept new things. They let issues mentally fester and are stoic. I likely don’t have this correct but I’ve had it explained to me in the past and it made some sense with horse behavior.
Parelli has a whole system to “label” right and left brained horses ( “emotional” or spooky horses versus “logical” and confident horses) and extrovert versus introvert horses. I don’t understand it well but I’m sure you can google it to understand how he classifies horses and decide for yourself.