Cooling System Fitting Removal Help Needed

That is a heater hose nipple. A pipe wrench will grab it, but the sides usually fall in if the walls have corroded too thin. Use penetrant and heat. Keep applying torque back and forth until you hopefully see it move, then keep flushing in penetrant and going back and forth to flush out the rust, then it should unscrew. I have unscrewed them even when the sides collapsed. Wrap Teflon tape on the threads of the new nipple so it comes out easy in 30 yrs.