I came up with another one: I think everyone remembers that Team Rocket guy from RBY with the underleveled Raticate, right? That got changed in FRLG, presumably because it was considered too strong (similar to Emerald changing the Milotic trainer in the rain route to a Kecleon).
Well, in Route 11 east of Vermilion there's an Engineer (called Bernie in the remakes) with two Magnemite and one Magneton all level 18, which in gen III can actually be harder than Lt. Surge if you don't have a Ground-type/a Dig user because they'll resist 90% of your moves. I fought him with a weakened team and I actually needed to SleepSeed and pray not to get Sonicboom'd to death because I just couldn't beat it fast enough. Like I guess you can argue it's there to train your Diglett or w/e for the gym fight but compared to
every other trainer (plus
the gym ones) that just seems out of place.