There are actually animals that sleep half a brain at once: all the marine mammals. A dolphin can't just fall asleep underwater (it will drown), so it sleeps half at a time.

There are creatures that sleep very little (Giraffes, etc.) But there's large benefits to sleep, and for most animals it's worth the cost, especially since you don't want to burn piles of calories during non-peak hunting/foraging hours anyway.