March of the Marlas
Everyone needs a spirit animal. Charlie Sheen has tigers. Lena Dunham has Taylor Swift. We have a baby rhino, obviously. In one of his support groups, our main character/ Jack learns to meditate and:
"Step forward into your cave. That's right. You're going deeper into your cave and you're going to find your power animal."
It's an adorable penguin, and it tells him, "Slide." Shucks. That's adorable.
But when Marla Singer shows up and puts a cramp in his style, suddenly his spirit animal is Marla. He goes into his cave only to find her sitting there sullenly. She blows smoke and repeats the penguin's mantra, "Slide."
Our narrator refuses to let Marla be his spirit animal (or even a part of his life) and he tries to get away from her until, in the end, he realizes that they are kindred spirits. He has a lot more in common with her than he does with a dang penguin.
There's also a point in the movie as our narrator really gets to know Tyler:
NARRATOR: He had a plan. And it stated to make sense in a Tyler sort of way. No fear. No distractions. The ability to let that which does not matter truly slide.
There's that word again: slide. And since Tyler is Jack, we guess that Jack's spirit animal (nope, not the penguin) must have gotten through to him.