Question: In the question; [Hollow-boned Humanoids](https://worldbuilding.stackexchange.com/questions/13619/hollow-boned-humanoids) Nathaniels humanoids were a flying mammal species called soarfolk. But let's say that I want a sapient mammal to have hollow bones but not fly.

These furred, sapient mammals are called Lokk, they evolved in the plains after leaving jungles. They are an average of 5"4 and 50 kg. The length of their arms(3.1 ft) Legs(2.8 ft) and tail(2.7 ft). They will be preferably digitigrade(though not a requirement) and of course they have hollow bones. 

How could a creature like this evolve in an earth like world?

Answer: 
Maybe while living in the jungles they were arboreal and the lighter animals were able to stay higher in the trees, or be able to 'flee' across smaller branches in the trees allowing them to live, as they 'ran' from predators. 

So like flying, staying up higher in the trees would encourage lighter bodies, but having hollow bones turned out to be more advantageous than a smaller frame, (like longer arms to help reach farther to grab the next branch.

The other might be if they were originally a flying species that started living in the treetops of the jungles and lost the ability to fly, but became more monkey like as generations came and went.
Perhaps there are poisonous gases created by underground bacteria in the forest floor which are fatal to your Lokk, and those gases dissipate in sunlight (i.e. concentrations would readily reach non-fatal levels as one approaches the top of the jungle.

If combined with plants whose branches retract upon sufficient jarring (i.e. Lokk who were heavier and disturbed the branches would fall into the gases), the Lokk would probably evolve into lighter, hollow-boned organisms, who could keep themselves out of the gases, up in the treetops.