The producers of the mentioned game initally planned Poison to be a full-fledged female, but due to the taboo against fighting a woman, Capcom of America requested that Poison be replaced in the American release with a male character.
Due to a miscommunication, Poison was replaced with a male transexual character instead, and her gender has been the source of some confusion and debates through the years. It should be noted that even her concept art mentioned she was a "newhalf" (shemale).
This fact and her sexy design have made Poison gain unusual fame. She also has cameos in several Street Fighter games and its crossover series.
The official word from Capcom is that in Japan Poison is still considered a newhalf, while in America and the rest of the world she is a post-operated transexual. Due to the addition of her partner Hugo Andore in SVC Chaos, SNK decided that they'd make Poison a full-fledged female.