Some animals are able to find a place to hide and sleep (like a burrow or den). Others are big enough to not worry about most predators.
Animals like birds, dolphins or whales let half their brain sleep. The rest of the brain keeps going, then the parts of the brain sleep wake up and the other parts go to sleep.
Some animals can basically lock their joints into place and happily stay like that without having to put in the effort to activate their muscles. They have other parts like special tendons or ligaments to support them. This can be advantageous if you’re a delicious looking prey that needs to be able to run asap when you notice a predator. However, they can’t enter deep REM sleep during this so they can’t do that all the time. They do need to lie down and sleep from time to time.
There may be some exceptions I’m not aware of
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