UFC's Mark Hunt -- Demands HALF Of Lesnar's UFC 200 $$$ ... He's A 'Cheater'

Mark Hunt is FURIOUS over Brock Lesnar's potential failed drug test ... DEMANDING half of B.L.'s UFC 200 winnings ... or he's bolting the UFC.

Hunt laid it all out on his website Friday night ... saying, "I've told Dana I want half of Brock's purse."

FYI -- Lesnar reportedly made $2.5 MILLION ... plus a percentage of the massive pay-per-view take.

"This is the third time I've had to fight a cheater in my career. The other two times I didn't ask for compensation, but this is the third time and I'm sick of them getting away with cheating."

If he doesn't get Brock's cash, Hunt says he wants out of his UFC deal. "I've told Dana that I want a release from the UFC if they don't sort this out."

The UFC has not yet responded to Hunt's ultimatum.