This seems to be a popular question, so let me explain how we reached the decision we did.
Many characters in Marvel are metaphorically connected to various creatures. Spider-Man, in particular, is known for having an animal-themed rogue's gallery. The question came up early when does a character get a creature type that is thematically connected to them?
The line we decided to draw was does the character have any elements of that creature type within them physically? For example, Peter Parker becomes Spider-Man because a spider bite infuses him with some spider DNA. He has spider powers because he is, from a genetic level, part spider. Dr. Octopus, in contrast, is not part Octopus. He has eight appendages, like an Octopus, but he is not, in any way, part Octopus.
Which brings us to to the Green Goblin. Norman Osborn becomes the Green Goblin because he takes a potion that alters his DNA. In the Ultimate version of the story (The Ultimate comics tackled familiar characters, but took a different spin on them), he literally becomes a Goblin. So, the big question was does the Green Goblin have Goblin DNA?
It's a hard question to answer, because the Marvel Universe is built on the real world, and Goblins don't exist in the real world. But Marvel has many fantastical elements. What does it mean to be a Goblin in the Marvel universe? The Green Goblin is such a big character, that his mere existence has warped what being a Goblin means. In some ways, the concept of what a Goblin is, in the Marvel universe, is shaped by the existence of Green Goblin.
On top of that, typal themes are very popular in Magic. When we can justify using popular Magic creature types, there's a lot of added play value.
The question of whether the Green Goblin should be creature type Goblin got raised early and was an ongoing discussion for the entire duration of the set's design. It was the source of hours and hours of discussion. I know this because I was involved in much of it.
In the end, we decided that there were good arguments on both sides. It did technically pass our DNA line if you were willing to say that the new DNA introduced into Norman was Goblin DNA (something supported by the Ultimate version of the character). So, the final factor was which would make for more fun game play? Would the Green Goblin being a Goblin be more upside or downside?
In the end, we decided it would be more upside, and made the choice to include it.