The Borgia plant was a poisonous plant native to the planet M-113. It was listed as a carbon group 3 vegetation, containing an alkaloid poison similar to the Earth nightshade family, with a chemical structure common to most M-class planets.
The M-113 creature taking the appearance of Nancy Crater blamed crewman Darnell's death on his ingestion of a Borgia plant. However, Spock discovered that the peculiar red mottling on Darnell's face was not among the symptoms of Borgia plant poison. Doctor McCoy subsequently assured Kirk that Darnell could not have died of alkaloid poisoning. (TOS: "The Man Trap")