If the target creature is an illegal target by the time Brightmare's ability tries to resolve, the ability won't resolve. You won't gain any life.
23-06-2020
wotc
Brightmare's ability can target a tapped creature. If the target creature is already tapped when the ability resolves, that creature just remains tapped and you gain life equal to its power.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.