Monster Sanctuary. It's like a good mix of Legend of Zelda and Pokémon, pixel-style sidescroller with surprisingly deep lore and a lot of quirky monsters with interesting designs to collect. Lots of puzzles, different biomes, interesting abilities, upgradable equipment, an interesting story, rival characters with actual personality (and character development throughout the story) ... all in all a well-made "monster collecting" adventure game.

It is also currently on sale in the Nintendo eshop, at least in Germany (5 Euro). No idea if it's the same world wide, but 5€ are more or less the same as $5 so it is pretty cheap for a game of that size and quality.