You need to unlock the third mastery skill (Canyon Jumping) for Raptor, then you will be able complete the appropriate heart and purchase the Springer. I don't remember if you get enough progress just by playing the story, so it's advisable to keep the overall goal in mind while you work on other stuff. (Springer's third mastery skill will be similarly required for the fourth mount, so once you unlock the Springer, I'd start working on it.) I will !!! Thank you !!!