Humanoids are part of the large portion of 'monsters' that characters encounter in their travels around the world. These are the races that are similar in shape to humans, but have a different mind set or live in a different environment that humans are typically not found in.
Mongrelmen are often also included in among the Goblinoid races as well. Gnolls are often considered the smallest of the Giant races or the largest of the Goblinoid races.
Humanoids that have a similar physiology are also grouped together like those that are called Dwarf though they may live in very different types of environments. Dwarf is often used as a generic term for many of the races that live in the Underdark.
There are other races that are allied with these Humanoid races but have a very different outlook on society or different physiology.
Some Humanoid races are grouped by type because they all come from a similar background even if not all of the creatures in that group are intelligent or have radically different shapes from each other.