Monster Boy is a member of the Legion of Super-Heroes.
Featured in these Comics
History
Arune Singh is from planetary Tor-Etto. Although "monster" is pejorative in some cultures, and Saturn Girl suggested that he could be called "Creature Boy", in his it is an earned title of respect.
He cannot transform into all kinds of creatures, only those of monstrous appearance, and he doesn't get to pick which monster form he transforms into in times of emergency.