Ecco Jr. is a bit of an oddity -- not entirely a canon game (unless you consider it happening during Ecco's childhood,) but it is a fun addition to the franchise. There are two versions of Ecco Jr.: the Mega Drive game, which saw a physical release in Australia and the US, and then the Sega Pico release which was titled Ecco Jr. and the Great Ocean Treasure Hunt! (or: 右脳開発シリーズ2 Ecco Jr. / Right Brain Development Series 2 Ecco Jr.). Both games are classed as edutainment, with a focus on exploration first and foremost.
Japan did see Ecco Jr. on the Mega Drive, but only through a download via the Sega Channel during August of 1995, so no physical carts exist.
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[PI01] Ecco Jr. And The Great Ocean Treasure Hunt [NTSC]
Title: Ecco Jr. And The Great Ocean Treasure Hunt [PI01]
Scans: Box Front | Box Back | Cartridge | Game 1 | Game 2 | Game 3 | Game 4 | Game 5