In the picture, the Pretty Little Liars star was shown wearing a bodysuit and a large, furry headdress, and the caption made a reference to the lion which was killed in Zimbabwe earlier this year (15) by an American dentist.
"Help! Can't decide on my Halloween costume this year! What do you guys think of this Cecil the Lion costume?," the post read.
The caption was later changed to remove the reference to Cecil, whose killing earlier this year (15) sparked international outrage.
Ashley has now returned to the social networking website to issue an apology and offer a donation to the World Wildlife Fund, and she also insisted she did not write the initial post herself.
"Yesterday's post was in poor taste and I absolutely regret all of the hurt that photo caused," she writes. "The caption that was posted was incorrect and my costume was not Cecil the Lion, when I saw that, I contacted my management who handled the post and had it immediately changed.
"I recognize that I am responsible for everything posted on my social media accounts and this was an unfortunate oversight. I am a supporter of animal conservation and I will be donating... to the World Wildlife Fund. I love you all and apologize (sic) if I let you down."
Dentist Walter Palmer sparked controversy in July (15) when it emerged he had shot and killed the popular beast during a $50,000 (£31,250) hunting trip at the Hwange National Park.