Singer Billie Eilish is in mourning on Sunday, revealing on her Instagram page that she lost her beloved dog Pepper.

The 21-year-old singer (born Billie Eilish Pirate Baird O’Connell) shared a number of current and throwback snaps of her and the beloved pooch.

‘Pepper. My life long best friend. I will see you again someday sweet girl,’ Eilish began in her heartfelt post.

‘You made it 15 years you f***ing beast. :’) I love you. Rest easy mama, I’ll miss you forever. This is a really hard day,’ Eilish concluded.

She also took to her Instagram story to share more snaps of her beloved pet on Sunday.

R.I.P. Pepper: Singer Billie Eilish is in mourning on Sunday, revealing on her Instagram page that she lost her beloved dog Pepper

Instagram story: She also took to her Instagram story to share more snaps of her beloved pet on Sunday

The loss of her pet comes just days after the singer opened up about writing What Was I Made For? for the Barbie soundtrack in an interview with Zane Lowe.

She revealed that both her and brother Finneas were invited to see a rough cut of the Barbie movie, though she wasn’t sure anything would come of it.

‘I went into it not thinking that anything would necessarily come of it. I was like, I just want to see this movie,’ she admitted.

‘This is so cool. I’m honored to even be thought of, whatever,’ Eilish added, while commenting on how Gerwig was apprehensive of showing anyone the movie.

‘She was just like, “It’s so scary to show anybody this. It’s like my baby.” She was literally like, “I don’t care. Please just, if you’re inspired, make something. And if you’re not, that’s fine. It’s fine. It would be my dream.”’

She added that she wasn’t sure if she’d write the song with Finneas, though the next day they kept playing around with ideas.

‘And we truly sat down and Finneas started playing piano, and… those first couple lyrics, “I used to float, now I just fall down,” just came right out…”I used to know, but I’m not sure now what I was made for,”‘ Eilish said.

Eilish added that she, ‘did not think about myself once in the writing process,’ though she realized after that she really wrote the song for herself.

Billie Eilish rocks the pink carpet at the Barbie premiere

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Opening up: The loss of her pet comes just days after the singer opened up about writing What Was I Made For? for the Barbie soundtrack in an interview with Zane Lowe

Song: She revealed that both her and brother Finneas were invited to see a rough cut of the Barbie movie, though she wasn’t sure anything would come of it

R.I.P.: Billie Eilish’s late dog Pepper

Hugs: Billie Eilish hugs her dog Pepper

Photogenic pup: Billie Eilish shares a snap of her late pup Pepper

Billie and Pepper: Billie Eilish shared snaps of her as a young girl with her dog Pepper

Pepper: Pepper hangs out under a picnic table in a throwback snap

Billie Eilish wears a ‘Cherubs from the Sistine Madonna’ shirt

‘I’m writing for myself and I don’t even know it. It is one of the most incredible things I get to experience in my life,’ she admitted.

Eilish wrapped up her Happier Than Ever world tour back in April, though it’s unclear if she’s working on new music.

The singer is diversifying though, making her professional acting debut in the Amazon Prime Video TV series Swarm.

She plays Eva, the leader of a dangerous cult that was inspired by the real NXIVM cult.