I question how you'd be able to forget Ledian. Personally, I think it's most easiest to forget a Pokemon that's a bit of a stand-alone. No evolution chain, no counterparts, absolutely no Pokemon to associate it with. In Ledian's case, it has an unevolved form, Ledyba (which should, on its own, be easily remembered as one of the early-game bugs in Gold/Silver, early bugs being something of a staple, as well as one of the premiere GS Pokemon that debuted in the 2nd mini-movie), and it also had a counterpart evolution-line in the form of Itomaru and Ariados (which are also early-game bugs, and anime-wise, the evolved form should be remembered for being owned amongst both Harley and Pokemon Hunter J. For the Gym Leader fans, Ariados is also used by Kyou and his entire family). In essence, there are four Pokemon associating Ledian, including Ledian itself, and four Pokemon out the 493 is bound to be a lot more noticed than one out of the 493 (it only helps that at least one of those four has had a number of memorable roles).
With Pokemon like Hariisen and Odoshishi, I can completely understand how they could get overlooked. They have no evolution chains, no counterparts to speak of, and to top it off, a majority of 2nd generations have been severely shafted in terms of showcasing, in not just the anime, but the entire franchise (anyone remember how difficult they made catching 2nd gens in the 3rd generation set of games? Hell, even in Gold/Silver/Crystal, they were overshadowed by an overwhelming amount of 1st gens, which were all made easily catchable in the wild.... god knows why, given the time machine feature). And it's not like they're anything that particularly sticks out neither, like a legendary (which, BTW, tend to have counterparts, namely the trios).