Celebrated bounty hunter Boba Fett is returning in he isn't actually a Mandalorian, Fett is a bounty hunter who wears Mandalorian armor, and he was considered one of the galaxy's most feared bounty hunters.
Boba Fett was introduced in returning in The Mandalorian season 2, played by Temuera Morrison.
This seems to fulfill a subtle tease offered in The Mandalorian season 1, episode 5. In "The Gunslinger," the Mandalorian headed to Tatooine for an episode filled with fan-service. He found himself going up against an elite mercenary named Fennec Shand, played by Ming-Na Wen, who had apparently arrived on Tatooine for a rendezvous of some kind. In the final scenes, the unconscious Shand was approached by an unseen character, whose boots made a clinking sound as he walked. Star Wars has associated those kind of spurs with Boba Fett.
The Mandalorian never revealed who Fennec Shand was meeting, but it seemed to be someone local, given she headed away from the spaceport and towards Mos Espa. What's more, "The Gunslinger" subtly hinted at some sort of Imperial presence on Tatooine, with the helmets of Imperial Stormtroopers on spikes, and Imperial credits now accepted at Mos Eisley Cantina. Ming-Na Wen is reportedly returning in The Mandalorian season 2, so presumably these mysteries will be resolved - and it now looks as though Boba Fett is involved in all this as well. He was most likely Fennec Shang's rendezvous all along.
Boba Fett has history with the Empire, working for Darth Vader himself in The Empire Strikes Back. It's entirely possible the galaxy's most feared bounty hunter is now part of Moff Gideon's Imperial Remnant; if so, he could well wind up dispatched to hunt down Baby Yoda. The interesting question is whether or not he'd be hunting alone, or as part of a group; Fennec Shand was a formidable assassin in her own right, and she'd no doubt make a potent ally for Boba Fett.