1. Sprites that Reward Amazing Accomplishments
Metroid is a go-to example, but Chrono Trigger also really ran with this idea.
 sprite - Chrono Trigger Special Ending.gif "According to Lucca, Frog as a human is a 'dish.' I can't speak for the culture of Guardia Kingdom, but in most culinary schools here on Earth, Frog as a frog is more of a dish."){kind=icon}
The challenge involved in seeing these sprites makes them rare—and their rarity makes them memorable. The Moonlight Parade dancer only shows her face (and her footloose skills!) after beating the game. Frog in his human form is “only” one battle tougher to get on screen, but some people will never see him outside of online sprite rips because their principles won’t let them meet the requirements. On the right are game versions of Akira Toriyama and Yuji Horii, who can’t be found unless players beat Lavos with only Crono and Marle or beat the souped-up, higher-stats, not-supposed-to-be-defeated Lavos at the Undersea Palace.
For a more modern and less 2D example, check out the ending of The Legend of Zelda: Twilight Princess. The character involved would have been striking even if the game had as little story as the original Legend of Zelda, but the tease of this appearance from the beginning helps further. Delayed gratification works.
Four more breeds, all discussed at greater length than the first one!