So I was meddling with the animation resource code to search for animations when I saw an animation with the code "a_loco_run_LFoot_x. I decided to tried it into a sim but then it played a running ani...
No, I'm fairly certain this animation is base-game. You're usually able to use it when your Sim is excited in some way. My Sim's husband started running like this when she was in labor, for example.