I was wondering if goblins were somehow just created like humans and elves, or if they were a cross-breed of some other creatures.
According to Tolkien, before the movies, Orcs and Goblins were not different creatures:
Orc is not an English word. It occurs in one or two places [in The Hobbit] but is usually translated goblin (or hobgoblin for the larger kinds).
—J.R.R. Tolkien, Preface to The Hobbit
Quoted from The One Wiki to Rule Them All.
The history of the Orcs is the real thing you need to look at.