The Ranger re-materializes on the ground, somewhat surprised at the beast's sudden retreat. Trying to escape? Trying to lure him into a trap?
Both. Many monsters, intelligent and animalistic, tend to go to ground when threatened or cornered. Their lairs are where they are strongest, where they are in home territory and anyone foolish enough to go in after them with anything less than a really huge shield and a huger fireball spell would die, either very quickly or very painfully...but equally hopelessly. Which means that if the thing got to its lair...it might be possible for him to take him out even so, but it would be easier if he could keep him out of it. Ideally, it would be a good thing if the monster were confined...but in its lair, the Ranger would be equally confined, and it would be a tough job to root it out. He turns ethereal again and blasts along close over the monster, a few more arrows close to hand and scanning the ground ahead for anything that looked remotely like...well...a monster-hole. Or a hole period along their path that was large enough to accommodate the thing.