You see a primal pack beast.
Notes
It spawns with the shape of a spider egg 
and changes into the shape of a Nighthunter one second later. If it's not killed in 20 seconds it becomes a Fungosaurus with full health. It's one of the 11 different types of Primal Pack Beasts.
It's part of the fight against The Primal Menace during the Primal Ordeal Quest.
Abilities
Can deal Critical HitsDamage Taken From Elements
Physical 
Death 
Holy
Ice
Fire
Energy
Earth
Drown
Location
Strategy
Kill them fast to avoid having to deal with the much more dangerous Fungosaurus.