The slimecat mount isn’t a season reward. It’s a participation reward. No different from the Corehound or the Obsidian Warbreaker. The latter of which was available in LFR.
Season mounts require heroic or mythic kills of the final boss. There is no new ‘season mount’ as the mounts from Sepulchre of the First Ones will be attainable until the pre-patch for 10.0 drops.