I know exactly what you mean.Its good to have fish like that,they add a little more interest and it's a novelty when they do make an appearance.Something I like are having a "sleeper fish"... those fish that are not readily apparent but if you sit and watch the tank for long enough they may make an appearance.
I actually have a farowella catfish (not sure if it's a species from Ecuador) that I always forget to include in my stock list - but my kids and I sit in front of the tank everyday and look for it.
I call my farowella "Waldo" as in "Where's Waldo?" Sometimes I'll go days without seeing him and then he makes a reappearance.