Will Halo have ambient life?

Possibly yes. One account describes the ambient life as being very big, perhaps large enough to ride on, and possibly intelligent.

The demonstration movie from E3 in 1999 featured two kinds of ambient life-- a large, dinosaur-like creature, and a herd of small, toothy animals.

However, neither has been seen in any builds of Halo since that time, and it is not known if they will be present in the Xbox release version of Halo or subsequent versions.