The noun "creature" comes from the verb "create" and refers to anything created (by nature). It is used as a word to describe anything living: most usually animals, but also humans ("You reckless creature!") and even extraterrestrials ("A creature from another galaxy"). The verb derives from the Latin verb "creare" with the same meaning. As it is a term encompassing more than just the animal kingdom, it would be wrong to associate it with the Greek word for "animal" (ζωο).
The word "πλασμα," transliterated as plasma and stressed on the first "a," is the direct Greek equivalent of the word "creature." It is a noun of neuter gender in the Greek language, deriving from the verb "πλασσω" (plaso), which means "form" or "create." As in English, it is used in Greek to describe living organisms, mainly animals, but also humans: "Εισαι το ομορφοτερο πλασμα" -- You are the most beautiful creature (meaning person).
Ον (transliterated "on" and pronounced as the similar English word) is the present participle of the ancient Greek "I am" (ειμι). It is actually the neuter gender of the participle, to distinguish it from the masculine (ων) and feminine (ο...σα) forms, both meaning "being" (as in "being the only child"). Therefore, "ον" refers to something that "is" (as in Descartes's "I think, therefore I am"), that exists, and is used in the same manner as "πλασμα."
Υπαρξη (transliterated "iparksi" and stressed on the first syllable) is a Greek noun of feminine gender, meaning "existence." It derives from the verb "...παρχω" (meaning exist) and refers to both the abstract idea and creatures that exist in nature. On certain occasions, you can also use it when referring to people: For example, "αιθερια ...παρξη" means "heavenly (exquisite) creature" and is used when referring to a beautiful woman.