The Legendary Mudkip (PMD Rescue Team DX)

It was a Mudkip that Ttar gave an apple to in the Buried Relic dungeon (on the 21st floor) in the 18th episode of the series [''GAME OVER DUNGEON!! | Pokemon Mystery Dungeon DX (Episode 17)'']. What made it legendary was that it had the rare quality Forge a Path. It is one of the best rare qualities in the game and Ttar had never gotten it till here - in the 99-floor dungeon where it is the most useful, since there are items like Sitrus Berries, Protein, Calcium, etc. in the walls.

However, the Mudkip fainted on the 62nd floor (off camera), and since Ttar had no Reviver Seeds left, was lost forever. Note that Ttar had not purchased the camp for Mudkip, so it would have been lost at the end of the dungeon anyway unless Ttar found a Wigglytuff Orb, which he did not in that episode.

Ttar did not get another Forge a Path-er until 2 episodes later (as 2 Electivire, both in a Mystery House) in the last episode of the series. And thus, Mudkip lives on in spirit forever.