Help: How do I make a flying mob look down when descending, and look up when ascending?

Just as the title says. Currently, the flying mob I made just ascends up and down like an elevator. I want it to look like the phantom when it's moving. Just to clarify, I've already have the AI already and it can move, I just wanna know how I can make it ascend and descend like the phantom. Or rotate it's model depending on the direction it's moving.