My friend gave me some mystery eggs, And one, a chansey with Seismic Toss came out! I checked its page on serebii, and it doesn't get Seismic Toss by level up or by egg move.
Can someone please explain??
It's legitimate; Seismic Toss can be chain bred onto Chansey in Sun and Moon. I assume you must have glanced over the listing for Seismic Toss on Chanseys Serebii page, as I checked it as well as both the Serebii and Bulbapedia pages for Seismic Toss, and it was listed.
Hope I could help!