Titanothon

Millenia ago, Saturn, Uranus, and Neptune were forged together by some mysterious force. Possibly aliens. Whoever they were or whatever it was, in the aftermath a new planet called Titanothon was created, which, for some reason, had properties quite different from what you'd expect a conglomeration of Saturn, Uranus, and Neptune to have. Perhaps the outside force did some modification before leaving the planet.

Titanothon is still like that. It also has many diverse moons.

Stats
Atmosphere:
 * Mass: 7.6×1026 kg
 * Volume: 2.2×1014 cubic miles or 9.4×1020 cubic km
 * 90% Hydrogen
 * 7.7% Helium
 * 1.1% Methane
 * 0.7% Water
 * 0.3% Ammonia
 * 0.2% Other