My problem's worse - for me, the mannequin never appeared in the first place! I bought Vlindrel Hall, the player house in Markarth. There is supposed to be a mannequin in the living room once you buy the living room furnishings, as I did before I went inside the house for the first time.
Unfortunately, my living room has the pedestal in the corner where the mannequin should be, but no visible mannequin. Trying to interact with the mannequin by pressing E doesn't work either, so it seems the mannequin truly doesn't exist, or has glitched itself into a place the player cannot reach or see. How might I fix this?