As seen in "Winter Forecast," gems can temporarily give creatures their powers. I think Rose did that for her cat that was mentioned in, "Maximum Capacity." There's also a picture of a cat that is tangled in pink yarn in the warehouse that means a cat was tangled up with Rose. I believe that Rose gave her cat the same power that it's in her room and also a pocket-dimension. She put bismuth inside after she bubbled it, the baby video, and everything else that's there. She made Lion as a guardian for Steven and by placing Bismuth(part of the Rose Quartz fusion) inside she knew he would feel compelled to do so.

I believe Bismuth was one of the secrets Rose kept from Pearl. Imagine it. Pearl has romantic feelings for Rose and considers Greg a fad that will pass. But a gem? She could never cope if Rose was with another gem! It would crush her! Rose decided the best way for Bismuth to exist and function as Steven's guardian, was that she had to bubble it and give it a temporary body(like the Big Bird). She gave her kitten/lion cub some of her abilities (possibly though a kiss on the forehead) and it became a temporary gem-like creature. It grew a pocket dimension and Rose was able to hide Bismuth inside.