Anyone who has sold very few stories professionally and/or hasn't been at it long.
Unlike neofan, which is rarely a term of opprobrium, neopro is often used to designate annoying professional newbies — new professionals who aren't full of themselves tend simply to be called pros, while fuggheaded authors may merit the label long after they've been published.
This is well illustrated by a story told about a new writer in the 1960s who was visiting Boston and dropped in on a NESFA meeting at Tony Lewis' house. At the meeting, club members happened to be collating Instant Message, the clubzine. One of the club members said "Welcome! Will you help collate?" The writer replied, "No, I'm a professional now, so I don't do things like that anymore." Whereupon Ben Bova came in from the other room and said "Isaac (Asimov) and I are out of page 6 — can you give me some more?"