The metal dragon bird
Billy the Butterfree, 5-19. He was just starting to be awesome when he died to a Trainer's Raticate in a rematch on Nugget Bridge when I was grinding for Misty.
Only death of that run...
To add to that...Babbie the Rookidee (Shield)
Ooh, boy, this must be the first ever gen8 death. Died while catching my Yamper. I was like, early game Electric mons almost never get Elecric STAB until later so I thought I would be safe. Jk, it had Nuzzle, and Rookidee dies.
Partot the Rookidee (Sword)
Died in the Wild area, against a Delibird. I was not paying attention to the fact it was higher levelled since it was just a Delibird, and then it kills my 2nd chance at owning a bird. Guess the game doesn’t want me to own a Corviknight.
Torecco the Wingull (Sword)
Another death to the Wild area, this time while catching my Stufful, who got a high roll on it’s 2nd attack. Poor Wingull couldn’t even make it past 3 areas from when he was caught.