Which planet's moon Triton orbits backwards?

Neptune. Triton goes round in the opposite direction to the planet's rotation, which no other large moon does - and a retrograde orbit is essentially impossible for a moon that formed in place alongside its planet. It must have been captured.

The likely origin is the Kuiper Belt, which makes Triton a close relative of Pluto that got too close to Neptune and was caught. That capture would have been violent: it would have wrecked whatever moon system Neptune had, and Triton's orbit would have taken a long time to circularise, with tidal heating melting the moon through in the process.

It remains geologically active. Voyager 2 photographed plumes of nitrogen erupting several kilometres up from a surface at minus 235 degrees, among the coldest measured anywhere, and much of the terrain is young and strange - a wrinkled expanse nicknamed cantaloupe terrain that appears nowhere else. Triton is also spiralling inward and will eventually be torn apart into a ring, in a few billion years.