Please get the idea

When your brother suddenly appears in your boyfriend's room (his room too) and you have no other choice than to turn into your animagus form and pretend to be just a random cat, but you don't know he's also an animagus and he's a dog so now he considers your bf a traitor, not for dating his little brother but for liking cats

He turns to his animagus to cope. Cat emotions are way less complicated than human emotions.

But he just... never turns back.

He spends years living as just a cat, hiding from both the death eaters and the order. So long that when he finally does turn back, it feels wrong. It feels unsafe.

Regulus Animagus form

Aka a small furry black kitten a very furry kitty

It's enough, they ease up for just a moment, and he takes it. His body curves, arms twisting, neck growing. He spreads his would be arms and flies.

A beautiful black swan risies into the cold night air, flying high, higher to the hole at the top of the cave.

Escaping for good.

Regulus Black escapes, for good. His family, his fate, his death. For Regulus Black was always supposed to live.