UPDATE: It's just been hotfixed. New spawns in the area (as of a few mins ago on a US server) are now appearing at level 23-24 as expected. So the following is now out of date...
I hadn't seen anyone note this yet, but I found that Malayan Quillrat, Malayan Quillrat Pup and Bandicoon battle pets that spawn in the Veiled Stair zone are level 1 and I could tame one with a level 4 rat
I presume that battle pets aren't assigned a hard level, but rather take on one based on the zone they spawn in. And it looks like they forgot the Veiled Star, so they default to 1.
I've reported the bug to a dev, so it'll probably be fixed. But I'm sure it's not much of an exploit to grab a level 1 one while it lasts
EDIT: updated the zone info. Originally I thought it was just the Ancient Passage, but it's all of the Veiled Stair.