It's just a bug in the spell book.
Those Mounts look fucking glorious! And are likely from (1-2) Specific Raid Tier(s), (1) PvP Reward/Purchasable, (1-2) Reputation Purchase, (1) some Hardcore Achievement/Legion Release window Purchasable with special Currency, (1) Mythic Dungeon final Boss rare drop (since CM's are gone in Legion). Just my guesses, not from any actual Blue Post.