Long story short, Shinobu (Kira's pseudo-wife) randomly found a cat in her basement and accidentally killed it with glass. Since this is JoJo, cat was a Stand user and got revived as a plant that shoots air bubbles. Kira beat it with his genius-level mathematics and decided to put it inside Killer Queen's stomach.

Because Araki wanted him to, the real question is why wouldn't he?

Eye Lad it’s asking why the box is there not how he got Stray Cat

Cuz why not

Stand made of energy

User can make it do things

One thing is make cabinet

It's the pretty cabinet mentioned in the song Killer Queen.

Ha ha stray cat go brrrrrr

Stands can likely shapeshift if user wishes so, like how Kira gave KQ a skull for face in its anime debut.

Hes pregnant

Bc Araki is Hirohiko