Not new species but new locality records. The first record of Least Shrew in my state (New Mexico), where it was unexpected, was brought in by a house cat. Cats are pretty good at finding overlooked small animals.
It would be interesting if there were any mollusk species known only from specimens found on the back of a carrier shell (Xenophora), but I don’t know of any.
Another aspect of this that deserves mentioning is that I’m using the word ‘discover’ here in the scientific context. But that has the effect of ignoring the fact that many of these ‘discovered’ species were well know to indigenous human populations before western science entered the room.
Probably the most famous example of this is the discovery of living coelacanths in in South African waters coastal waters in 1938, where the fish was already known to native fishers there, and also soon afterwards off West Indian ocean islands where again, the indigenous folks were already fully aware of their existence.
I think, at least I have heard, that science has finally started to really clue in to the value of the incredible natural knowledge that many indigenous people have developed over the centuries and that many, many new species have been catalogued as a result of these evolving partnerships between science and traditional knowledge.