Just one day after releasing iOS 12, tvOS 12, and watchOS 5 to the public, Apple has released the first beta builds of iOS 12.1, tvOS 12.1, and watchOS 5.1 to developers for testing.
The new betas are available as downloads from the Apple Developer Center, or as over-the-air updates for devices with previous beta versions installed.Public beta versions of iOS 12.1 and tvOS 12.1 should be available within the next few days or so.
The build numbers are 16B5059d for the iOS 12.1 developer beta, while tvOS 12.1 is build 16J5575c, and watchOS 5.1 is build 16R5559d.
Apple strongly warns against installing beta software on daily drive devices, and instead recommend only installing betas on devices designated for testing purposes only. Plus, backup, backup, backup. (DUH!)