A Croagunk backpack!
I believe it's official merchandise, as I've seen the promotion piece it was included in (that I sadly can't find now). However, I haven't seen many sellers offering the item, and those that are look a little less than reputable.
I was wondering if anyone in the community had any information on this little bit of merch, and/or if they had one for sale. As always, thank you for your time and have a great day!