Pretty sure Pete E posted about those. I need to get one. Seems like when I kill something nobody is around to help lift.
Yes, I did mention them here a while back after seeing a guy in our Club use one...
The largest ones I have come across are rated to 250lb, but I am some you can probably get heavier duty ones if you did some searching.
That said, if a person is wanting to lift game in excess of 250lb, you'll probably want a block and tackle arrangement rather than a single pulley, as the single pulley offers no mechanical advantage...