And lo and behold, both are also hidden in Serebii's Attackdex.
Both are physical Ground-type moves with 90 power and 100 accuracy, but especially interesting are their added effects. Thousand Arrows hits fliers, levitators, Magnet Rise/Air Balloon users, etc. and neuters their Ground immunity on top of that. Meanwhile, Thousand Waves traps whichever Pokémon is hit. Clearly both are very dangerous moves that could turn the metagame upside down if they're ever given to good Pokémon, especially many of them.
Now the big question is, who are these meant for? You'd expect such rare Ground moves to be given to Ground-types, but none of the event Pokémon are, and neither are Eternal Flower Floette and Mega Lati@s. This leaves one logical conclusion... Zygarde. Whether it gets an alternate form or not, these moves would certainly make up for the disappointment Land's Wrath is. Oh, by the way, guess which move is a physical Ground move with 90 power and 100 accuracy?
Edit: Turns out this is only news for me. No one thought it'd be a good idea to tell me?