Lotr and Harry potter are extrodinarily different...
The trolls for example are for starters in the LOTR are made from stone 😄 and in many fantasy fan fictions very varying in looks.... some are tall and slender and earthlike while in lotr and Hp they are hefty and bulky.
Goblins in HP are intellegent and drawn to wealth in HP and are more like gnomes rather while in Lotr they are ferocious fighters similar to orc. Elves are peobably the greatest in difference between the two stories for obvious reasons 😛 in HP they are the generic Santa's size munchcins while in most other fan fics and lotr they are similar to humans. HP seems to be more magically inclined while everything else including Lotr is mystical and mysterious with great forces binding everything together.