Legendary Pokemon Gen 1-4

This guy probably thought so because it's encountered like a legendary, there was only one, it changes forme which is rare in a non legendary those days, and its sinnoh dex number was just before the lake trio so it was grouped together with legendary pokemon for the longest for a while because of multiple coincidences, and because it wasn't a normal encounter until XY many others believed that until that day. True trainers like Colress had one in BW but those games were littered with trainers that used legendaries.