I'm leaning towards it being fake. Koromori doesn't have a nose that looks like a chestnut. It's clearly a heart. In only the early art it looks like a chestnut. And according to CoroCoro scans, Koromori is its official name, too.
Oh and Nezumimi, as mentioned, is Minezumi (confirmed I think?), which points towards it being fake for not having the correct name either. And that was only mistaken by everyone else, not someone who actually works on the Best Wishes team. And plus, the green toy wasn't even confirmed as real anyways - so it can't be that Pokemon either, as it is mentioned.
More reasons for it to be fake than not.
...I'm finding way too many flaws in this. XD;