From Sega Retro
The Sega Pluto is an unreleased video game console developed by Sega during the 1990s. The console's existence was kept away from the general public until April 2013, when a former Sega employee leaked details of the system onto the internet.
The Sega Pluto would have been a cost-reduced Sega Saturn with built-in NetLink Internet Modem (making it one of the first consoles to have internet support straight out of the box). It is unknown exactly how far the project was carried through before it was cancelled - only two prototypes were ever produced (Pluto 01 and Pluto 02, respectively), both presumably from around the 1997-1999 period.
Both Pluto prototypes have been found, though they lie in the hands of collectors. Both share the same shell but have minor aesthetic differences - Pluto 02 has the text "Sega Saturn" printed on the unit, and "Pluto 02" printed on the underside, while Pluto 01 lacks both of these things. Each have a North American BIOS which may suggest it was only ever intended for that market (most likely due to the NetLink technology being scarcely seen outside of this region). Pluto 01 also has a faulty disc drive door which does not fully close - as to whether this was a manufacturing defect or something picked up since is not yet known.
According to a subsequent auction of Pluto 01, the system is 9 inches x 3½ inches x 14 inches in size, with an estimated weight of 5lbs. These are similar dimensions to a Sega Mega Drive 2 connected to a Sega Mega-CD 2.
"Pluto" was likely just a code name and was unlikely to be used in marketing.