Warning! The following contains spoilers for Avengers: Endgame. Read at your own risk!
Avengers: Endgame's battle sequence at the end was massive. So massive, that if an unexpected character happened to help out, many in the audience may have missed it. Fortunately there were some who happened to catch an entirely unexpected Guardians of the Galaxy cameo, as a character from the movie popped in to lend a hand in the Avengers' time of need. Howard the Duck, of all people, showed up to fight for the world alongside the universe's mightiest heroes.
For those attending another showing in the near future, Howard can be spotted amongst the many warriors showing up when Doctor Strange, Wong, and the rest of the sorcerers start opening portals. When Hope Van Dyne is spotted, viewers should move their eyes to the blueish alien to her right, and then follow that character's gun downward. There's Howard, in the distance, looking a lot like he did in his cameo scene following Guardians of the Galaxy.
Howard's packing heat in lieu of a drink this time around, and he's looking poised to kick some Earth invader ass when the signal is given. His contributions beyond that are unknown, as he doesn't get a big action sequence to show what he's up to like other key characters in the Marvel Cinematic Universe. Apparently, the Russos didn't find it pertinent to zero in on what one of the MCU's most bizarre characters was up to.
Here's the good news: There's still a chance we may learn what role Howard played in the battle. That's all thanks to an upcoming Howard The Duck television series which will feature the Marvel hero trapped on Earth. Provided this series takes place post-Avengers: Endgame, there's certainly an opportunity for the series to show that scene from Howard's perspective, or have him recall something from the event. Hell, this may even be the event that strands him on earth!
Whatever the case regarding the future series, it's been known that the Russos have wanted to include Howard in the action since Avengers: Infinity War. According to Joe Russo, the original idea was to have Peter Quill go to Howard and ask him about the whereabouts of an Infinity Stone while he was in the midst of a poker game. That would've been great to see, although one could argue this is better.
The real question now is, what was Howard The Duck up to before his arrival? The Russos confirmed he survived the snap, so there could be some deleted scene or one shot in the future that outlines what Howard was up to in the time between Infinity War and Endgame. If not, at least it's good to see him alongside the rest of the good guys, and possibly making a case to become a future Avenger?