I have that problem with a mare at my RS - Beauty, a approx 16hh heavyweight cob. She's a big girl, and is not a good horse to choose to have an argument with
She's fine with her fronts, will let me pick them up and she will support most of the weight. The backs, however, are a totally different story
First, she will constantly move round the stable, refusing to stand still unless she's wedged with her back end in a corner, making it impossible to pick feet up. When she is finally standing still (which can take any time upto around 20-25mins, even when she's tied up
), she will refuse to pick her back feet up. So we fight over her feet for a while, the eventually she will pick them up. She will then not support her foot AT ALL, and expects the unfortunate person picking them out to hold them up for her, and with her being so big, they are VERY heavy
So then, I try supporting her feet on my knee, or holding them by the toe. If you don't have a firm grip on her foot and aren't holding it up for her, she will snatch her leg upwards, then slam it down as hard as she possibly can (my friend's hand was in the way one day... that was painful
)
Just really can't win with her. It kills my back doing her feet, and you need at least half an hour free to guarantee that you can get her feet picked out in one go