For most of my campaigns, the basic premise is that humans and humanoids are the norm. So other humans, (dark)elves, dwarves, orcs, goblins etc. are the most common enemies. The main fantastic enemies comprose of several 'types' of monsters with a human(oid) basis:
were creatures (the cursed, these are humanoids living under some curse which is passed on from generation to generation)
mindless undead (the lost, usually the result of necromancers, sometimes other magical or divine effects)
intelligent undead (the corrupted, usually from some pact or ritual made by the humanoid (vampires, liches etc.) or revenant behaviour from humanoids who do not want to give in to death and refuse to pass on)
half demons/devils (the damned, which are VERY rare, the result of willful breeding programs with summoned demons or devils by certain cults)
shapechangers (the hidden, dragons who live in human form, dopplegangers etc., strange beings who survive by fitting in as much as possible)
other 'common' adversaries are constructs of all types (from golems (magical) to mechanical constructs) and summoned monsters (mainly elementals and devils, demons, daemons etc.)
Other 'common' adversaries are the offsrping of (magical) breeding programmes resulting in creatures such as bulette and other strange nasties
In the vast wilderness strange and fantastic beasts can exist, but these are special and rare.
So in essence, only dragons are relatively 'common' fantastic creatures, but this is usually the remnants of the age of dragons who learned to survive against the tide of human(oids) by 'joining' them, and living among them in shapechanged form. At lower levels, the PC's will not even know they exist at all, they existance is generally a myth and the truth is known only to a handful.