Buttocks basically refers to either of the two meaty protuberances for the back and lower part of the human trunk.
"The nurse asked me to show my buttocks so she could inject it."
"Buttox" is an abbreviation or the phonetic transcription of buttocks. When you use "buttox" you are referring to both the buttocks and not a specific one. If you say, "I have an itch in the left buttox", you would be wrong.
"Buttox" is NOT accepted to be used as an English word in formal writing.
Do not confuse it with "botox" which is an abbreviation for Botulinum Toxin, a medicine used for tightening facial muscles and treating some medical conditions.