I don't know what the rarity of this item is - in fact I didn't even know it existed until I got it. I presume it's a new addition to the Winter Veil world event this year. I got it on my second go - I took Atherya out, did the daily, wondered why I was doing it because I thought I had everything, but I had already gotten Spinegrinder to Tirisfal Glades, so I decided to just carry on. When I came back, I turned in the quest, opened the box and there was a flash of blue! The Rotten Helper Box.
The little freak has a board with a nail in it, and he mounts up on a reindeer when you mount up. Sadly, it does not fly, which I find a bit odd to be honest, but it's quite awesome that he gets on a little reindeer! Good luck everyone else on obtaining this pet!