If you truly refer to an Outsider, that can only be Ygorl, the Slaad Lord of Entropy, and the closest thing Limbo has to a ruler. It is huge, chaotic evil, and has the appearance of a blackened humanoid skeleton. Its weapon of choice is a scythe. It was originally detailed in the 1981 Fiend Folio, published by TSR long before 3.5e was even imagined.
There is an unofficial 3.5e update to Ygorl available here, by the noted Planescape expert afroakuma.
Beyond Ygorl, however, the 2004 3.5e book Libris Mortis includes the entropic reapers, which are truly undead rather than Outsiders, but they are native to Limbo and share a similar appearance with Ygorl. Libris Mortis does not mention Ygorl, but the entropic reapers’ appearance and association with entropy is enough to link them. They are notable for their plane shift ability, and their proclivity for spending a lot of time on the Material Plane rather than Limbo.