We’re looking at Jigglesworth Sr., the mount you’ll get upon completing the Shadowlands Season 4 meta-achievement.
Fates of the Shadowlands Raids (Shadowlands Season 4 Meta Achievement)
You must complete all 3 Fated raids in Shadowlands Season 4 on Normal difficulty or higher to receive Jigglesworth Sr, the slime cat mount. Fated raids will be on a weekly rotation, so you won’t be able to get the mount before August 16, 2022, when Fated Sepulcher, the third raid necessary for the achievement, becomes available.
Flavor text: It’s unclear how the saddle stays atop this formerly feline friend, but somehow the House of Plagues made it work. Science.