Hi, fululu!
I assume you meant a bag to put the baby stroller in when you get on board???
Most airlines now do not allow baby stroller on board, unless the flight is almost empty and they can find an empty overhead bin or somewhere to secure the stroller safely inside the cabin. Most of them tag the stroller at the check-in counter, but allow you to drop the stroller of at the aircraft's door. You then get on board, and the stroller goes down to the belly. They will return the stroller to you at the aircraft door when you deplane. I personally love this system, as I don't have to look for a place to put the stroller away.
I am lucky enough to always have the stroller back in the same condition I dropped it off. At least 2 friends of mine didn't. But the airlines fixed the strollers for them.