The ability of Tyrannical Pitlord that allows you to choose a creature isn’t a triggered ability and doesn’t use the stack. Players can’t respond to your choice of which creature it’s affecting.

If Tyrannical Pitlord enters the battlefield at the same time as another creature, that creature can’t be chosen.

If the chosen creature leaves the battlefield, you can’t choose a new creature.

If another player gains control of the chosen creature while Tyrannical Pitlord is still on the battlefield, that creature will continue to have flying and get +3/+3.

If Tyrannical Pitlord’s leaves-the-battlefield triggered ability resolves while its controller is not the same as the controller of the chosen creature, that creature will not be sacrificed.