So, what WAS the bird Ash saw at the end of episode 1?

This is something that's bugged me since I was a kid. At the end of the first episode, Ash sees a golden bird fly over the rainbow, and the Pokedex says it doesn't know what pokemon it is. In the following episode, while at the Pokemon Center in Viridian City, Ash sees a fresco of four legendary pokemon- three are obviously the legendary birds (Articuno, Zapdos, and Moltres) and the other is an Arcanine (for some reason- it's not a legendary pokemon, though in Red/Blue/Yellow the pokedex says its type is "Legendary"). Ash focuses on the blue square of the fresco (Articuno) and makes a big deal about how that's the pokemon he saw. So, Ash saw Articuno apparently.

...Except that it wasn't Articuno. The pokemon he saw was clearly Ho-Oh, from the Gold/Silver/Crystal series.

So what's the deal? Had Ho-Oh even been invented yet when this show was made? And why did they make such a big deal of making it seem like Ash had seen Articuno when he clearly hadn't?