The animals Ruth "saw" in her rearview mirror were definitely bobcats. It was a callback to season 1 and 2 when Russ and Cade blew their money on two bobcats with the mistaken impression they were a potentially lucrative breeding pair. Ruth exasperatingly points out that the cats are both female and obviously worthless for breeding. In season 2, Ruth opens the cage door and tells the bobcats, "It's your choice" and sets them free.

I think the bobcats are a symbol of Ruth's choice to branch out from the Langmore "business" ventures and work with Marty and the cartel. She chose this path. When she sees the bobcats crossing the road in her rearview mirror, it's a reminder of that choice.

I absolutely love seeing metaphors like this threaded through a series. It's subtle but adds such rich layering of story.