Solus could be many things, calm, polite, observant, but brave? brave he never was. 

He wouldn't say he's a coward, but rather that he cares a lot about his own well-being, and who wouldn't care about their own survival!? 

So it's no surprise that when sent to search for a treasure chest, and saw the remains of others sent on the same task, he would run away... he would never set his hooves on something that would cost him his life!