Been to Thailand a couple of times and I always thought Buddhist monks were holy lifelong devotees that never did any wrong. Turns out that there is parts of the country where reformed prisoners have the option to join a monastery as a monk to serve their sentence. So in many places, those tough looking monks you see are actually gang members and criminals serving out a sentence .... some of them are honestly reformed, some are working on it, and some are just gaming the system.
Some monks train their bodies either as part of their prayer or meditation, or train in order to be able to pray or meditate longer. They don't usually end up with arms that look like that, though.