Ru is a single-syllable name that functions as a nickname form in multiple traditions: short for Ruby, Ruthie, Rufus, or simply a standalone monosyllable that works acoustically as a pet name. On a female dog at this registry rank, it's most likely a clipping of Ruby or a personal nickname that an owner transferred directly from human use.
The Nickname Clipping Pattern
Single-syllable pet names often emerge as clippings of longer names: Max from Maximus, Bo from Beauregard, Ru from Ruby or Ruthie. The appeal is purely acoustic — one syllable, clear vowel, easy to call. Ru has a soft onset that makes it gentle rather than commanding, which suits small female dogs and cats particularly well.
RuPaul's Drag Race
RuPaul Charles, simply Ru in casual reference among fans, hosts RuPaul's Drag Race, one of the most culturally impactful American television shows since 2009. A dog named Ru in a household that watches the show is almost certainly named for the host.
The Counter-Reading: Too Clipped for Some
Ru at one syllable can get swallowed in a noisy environment. Owners who want something more robust may prefer Ruby and use Ru as the daily nickname. Both work; the registry just captures whichever one the owner wrote down first.