This is a paid asset, but now you can download Alien Planets Vol.2 Free.
Detail this asset from Unity Store: Original Link
Alien Planets Vol.2 v1.0
“If possible, please buy the package to support the developer”
Assets to create your own Alien Planet environments! The assets are optimized for mobile and VR.
Includes 4 examples scene to get you started:
_All props, creatures and terrains assets are using only two textures/materials.
_Include 4 different skyboxes
_4 alien creatures with an idle animation and 3 skins each.
_Structures, terrains, cliffs, vegetation models, etc..
_Models range from 40 to 950 triangles.
_Particle effects: Ashes, Acid clouds, bubbles, volcano, gas cloud, rays, etc….