Fusions/Hybrids (My own interpretation)
For my own Pokemon world that I have for my characters, I created this explanation/definition and distinction of hybrid and fusion Pokemon xD I honestly have not seen anyone distinct these different terms themselves in their own Pokemon stories or design ideas so I came up with my own!
This is more of a piece of lore rather than literature haha
In my own pokemon universe there are hybrid and fusion Pokemon, but the two are not one and the same. Both are incredibly rare, however.
As many Pokemon fans know, two Pokemon of different species within the same egg group can breed with each other. Offspring is always the same species as the mother pokemon (with the exception of ditto). In my universe, Hybrid Pokemon are an exception to this. These Pokemon appear as a mixture of both it's parent's species, becoming a whole new different one of its own. In rare cases, some of these hybrids actually become normal species, but because of the rarity of having hybrid offspring, this outcome is very unlikely.
Fusion Pokemon are more special and rare. Because of their rarity, there are many Pokemon that don't even know how they come to be. Similar to hybrids, these Pokemon are a mixture of two other species, but unlike hybrids, these two different species are of opposing egg groups. This makes their whole existence one big mystery. There are several theories on this matter: Two Pokemon from opposing egg groups were somehow magically able to have an egg, the Pokemon is some sort of result of distant science or magic, or fusion Pokemon is an actual fusion of two different individuals. No one can say for sure.