Fel-Spotted Egg is a new item that added in Patch 7.3. Drop from Panthara rare spawns. after 5days it changes into Cracked Fel-Spotted Egg with a chance to contain mounts, pets, and junk:

Aged Yolk
pet: Docile Skyfin
pet: Fel-Afflicted Skyfin
mount: Felglow Mana Ray
mount: Scintillating Mana Ray
mount: Darkspore Mana Ray
mount: Vibrant Mana Ray
Fel-Spotted Egg drops by the rare elites of the Agus map.
Antoran wastes: 64.3 48.2 Varga
Krokuun: 70.5 33.7 Naroua 
Mac'aree: 38.7 55.8 Kaara the Pale 
Mac'aree: 44.2 49.8 Sabuul
These elites can be picked up once a day, as the same, you will get artifact power and Veiled Argunite by killing these rare elites. 
Fel-Spotted Egg is not the only one that you can carry more at the same time, which have greatly increased the chance of the mounts containing. You could buy the 2 pets in action house with wow gold. This will be a good choice for pet lovers.
Fel-Spotted Egg has the chance to be dropped., so does the mounts. Looking forward to your good luck.