Plot hole w/ Sam in s2 finale?

I don't want there to be one lol I love the show so much already. But Sam made it clear earlier multiple times that he needed an animal as a guide or mirror or copy, something to base his transformation on. How on earth did he transform into a bull without another one around?!

(side note: i really like that Tara's mom was the one and only 100% human person who was around Maryanne doing her thing at some point and still never turned. We have heard from Tara about, and witnessed, her being a mentally weak person many times, but she was the only one strong enough to resist Maryanne! I mean, she probably just wasn't essential to the plot, but I like to think it still counts.)