Ettin is the Old english equivalent of Giant
Ogre was another term for large monster/demon/giant - (add stuff about Orcus and Orcs here)
Ogre Mages - derived from Asian concepts of Ogres (demons)
Overall most of the Giants are derived from myth (giants are very common across the world)
Lizardmen - Snakemen were known historically as were those with human heads and reptile bodies. However I beleive Lizardmen were derived from pulp fiction and early facination of dinosaurs and lost worlds
Locathahs - fishheaded people are mentioned in some myths. but the locath specifically I have no idea
Merfolk - common european myths a type of fey, maybe derived from the Greek Sirens
Bugbears - another archaic english (as opposed to old english

) word for demon/ogre
Hobgoblins - Hob was a small grotesque fairy that caused mischief. I think Hob originally refered to an unshaped lump of iron ore and the various associations derived from that. NB
Tolkien states in his book that Orc is the hobbit word for Hobgoblin.
Blues - a DnD invention a goblin with psionic abilities