Along with iOS 12.1 developer beta 1, Apple released watchOS 5.1 beta 1 on Tuesday. Apple’s release notes are thin, so it looks like this is mostly a bug fix and performance improvement update.
Apple released watchOS 5 for Apple Watch Series 1 and newer on Monday. The update added new watch faces, new workout types, Walkie Talkie two-way voice chat, raise to speak Siri support, and more.
watchOS betas are different from iOS betas in that there won’t be a matching public version. Only developers get access to watchOS betas. You can sign up at Apple’s developer website.