Astronomers have posited an interstellar origin [bbc.com] for (514107) 2015 BZ509 [wikipedia.org], the first example of a retrograde co-orbital asteroid with one of the solar system's planets (Jupiter):
An asteroid in Jupiter's orbit [Note: actually orbiting the Sun and crossing the orbit of Jupiter] may have come from outside our Solar System, according to a new study. Unlike 'Oumuamua, the interstellar object [bbc.co.uk] which briefly visited the Solar System earlier this year, 2015 BZ509 (affectionately known as BZ) seems to have been here for 4.5 billion years. This makes it the first known interstellar asteroid to have taken up residence orbiting the Sun.
It is not yet known where the object came from. "That's what we need to figure out next," laughs Dr Fathi Namouni from the Universite Cote d'Azur, one of the study's authors. "Because 'Oumuamua was just passing by... it's not that difficult to go back and pinpoint where it came from," he told BBC News. "BZ reached the Solar System when it was forming, when the planets themselves were not exactly where they are now. So it's a little more tricky to figure out where it came from."
An interstellar origin for Jupiter's retrograde co-orbital asteroid [oup.com] (open, DOI: 10.1093/mnrasl/sly057) (DX [doi.org])
Related: Possible Interstellar Asteroid/Comet Enters Solar System [soylentnews.org]
Interstellar Asteroid Named: 'Oumuamua [soylentnews.org]
ESO Observations Show First Interstellar Asteroid is Like Nothing Seen Before [soylentnews.org]
Breakthrough Listen to Observe Interstellar Asteroid 'Oumuamua for Radio Emissions [soylentnews.org] (none were found)
'Oumuamua Likely Originated in the Local Association (Pleiades Moving Group) [soylentnews.org]