A captured wandering planet
Very like.
It would be most interesting to somehow obtain a sample, and compare the age of its material with that known otherwise.
The good scientifically determined age of our solar system, derived from a variety of agreeing phenomena, is ~5.5 billion years. This is the maximum age of any material (meteorites) that we can physically derive from it. Similar age derivation of the universe however, is ~13.5 billion years. So where is the difference?
If this object in fact orbits "the Sun backwards", it's possibly a remnant of earlier stellar system development, and thus would present good age-test of that which to now, has only been ascertained.