my ponytail plant leaves are turning yellow
Sometimes overnight they're turning yellow.
The soil is probably too wet. Ponytails don't like to sit in damp soil at all. If you have it in low light, get it into higher light right away -- just not direct sun. Try putting it where a fan can blow on it gently. The purpose is to dry it out as quickly as possible. You might even try removing it from its pot, setting the rootball on some newspapers or paper towels, and leaving it like that for a few days. In the future, don't water it until the soil in the bottom of the pot is almost dry. Use some kind of tester, like a moisture meter or a pencil you can stick in then feel for moisture. If it gets a little too dry, no harm done. The swollen base is a water storage facility.