-ist

(Greek > Latin: a suffix; one who believes in; one who is engaged in; someone who does something)

plagiarist (PLAY juhr ist) (s) (noun), plagiarists (pl)
Someone who purloins or utilizes the words, writings, or ideas of another person, and passes them off to others as his or her own creations: A reporter was accused of being a plagiarist, or a literary thief, when he failed to give credit to the person who wrote the original report.

Some musicians are plagiarists who take something composed by an old musical master and decompose it.

—Based on a quotation by Evan Esar in Esar's Comic Dictionary
planetologist
1. Someone who studies the planets including their origins and compositions.
2. An expert or specialist in planetology.
platformist
platitudinist
plebicolist (s), plebicolists (pl) (noun forms)
1. Someone who flatters, or courts the favor of, the common people; a demagogue.
2. Giving the appearance of being a friend of the people; an orator who appeals to the passions and prejudices of his audience.
plebiocologist
Someone who who flatters the common people in order to get them to follow his or her viewpoints and to get the people to take action in accordance with what is being promoted.
pluralist (s), pluralists (pl) (nouns)
Someone who holds multiple offices; especially, a clergyman who holds more than one ecclesiastical benefice.
plutologist (s) (noun), plutologists (pl)
Someone who is a specialist in the study of the wealthy and their powerful influences: "For her advanced umiversity degree, Karin intends to interview several plutocrats so she can become a qualified plutologist."
plutonomist (s) (noun), plutonomists (pl)
Someone who specializes in the political economy of the wealthy: "Mr. Mark was recently appointed the plutonomist for a prestigious business magazine."
podiatrist, podologist
1. Someone who practices podiatry (diagnosis and treatment of disorders of the feet), a chiropodist.
2. A specialist in podiatry; formerly called chiropodist.
podologist
A specialist in the treatment of feet.
pogoiatrist, pogo-iatrist (s) (noun); pogoiatrists, pogo-iatrists (pl)
A medical doctor who specializes in the treatment of ailments caused by beards.
pogonologist (s) (noun), pogonologists (pl)
1. Someone who specializes in the subject of beards.
2. A barber who is a specialist in styling beards.
polyamorist (s) (noun), polyamorists (pl)
Someone who has more than one open romance going on at the same time: It was difficult for Mr. Green, as a polyamorist, to keep his different romances a secret, especially to keep the knowledge of his private affairs away from his wife!.

It should be noted that "poly" means "many", not "more than one".

—From new words and senses from the new 2006 update of
Merriam-Webster's Collegiate® Dictionary, Eleventh Edition.
polyandrist
1. The marriage of a woman to several (more than two) men at the same time.
2. A woman who practices, or favors, polyandry.