That's what J.J. Watt just revealed he received from a fan on Thursday ... as the future Hall of Famer prepares to play the last NFL game in his legendary career.

33-year-old Watt shared pics of the "fan mail" on social media -- showing the box the stuffed badger came in, along with the animal, resting comfortably on a chair in the Cardinal's locker room.

"I have received a lot of wild fan mail over the years," Watt tweeted, adding, "This package that arrived today is certainly being added to the list."

As you know, Watt played college ball at the University of Wisconsin ... who are, of course, the Badgers.

Watt has presumably seen an uptick in fan mail ... seeing that the defensive star announced in December -- he intends to retire after the season.

"Koa's first-ever NFL game," Watt said alongside a photo holding his baby ... "My last ever NFL home game."

He continued ... "My heart is filled with nothing but love and gratitude. It’s been an absolute honor and a pleasure."

If this is Watt's last season, #99 -- a 5x All Pro and member of the All decade 2010s team -- will go down as one of the most dominant forces to grace an NFL field.

J.J. has also done a ton off the field ... and even won the 2017 Walter Payton Award, given to the player who has the biggest impact away from football.