-ist

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

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 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.
polychromatist
Someone who practices the art of using many or various colors in painting, architecture, etc.
polyculturist
Someone who cultivates a variety of crops or species.
polydemonist, polydaemonist
Someone who believes in many evil spirits.
polydiabolist
One who believes in many devils.
polygamist
Someone who commits polygamy or who has multiple marriages.
polymathist
1. Someone who is knowledgeable in a variety of subjects.
2. A person of great and diversified learning.
3. Someone of great learning in several fields of study.
polyphagist
Anyone who has an excessive appetite or who eats too much.
polyposist
A hard drinker; one who drinks excessively.
polytheist (s), (noun), polytheists (pl)
1. Someone who believes in, or worships, many gods or more than one god.
2. People who have beliefs in more than one god, usually a belief in many gods.