This article could easily start out as a kind of chicken and egg joke, like "Which came first... the camera or the lens?" I should add in 'legacy' or 'vintage' before the word "lens" to be more in keeping with the title.
Internal mirrors and pentaprisims started disappearing from cameras somewhere around 2008-2009, and makers offered lens mount adapters for their new mirrorless designs with the new mounts they used. Most of these adapters were produced by the OEMs themselves, and provided a quick path for brand loyalists to use their existing lenses on these new digital models, or even a conversion to a new body with a differently scaled sensor. Sony's full frame Alpha and SLT series and the NEX line offered this - or Nikon's D series (both FX & DX) when the Nikon 1's were introduced.