This Item/Block has a Hotbar and Inventory sprite that is an enlarged Fox Spawn Egg. It's not game-breaking and doesn't affect functionality in any way that I've tried. I've tested this out a bunch and there's a couple ways to get it. It's only obtainable via Warped Nylium by any Pickaxe in while in survival, and TNT regardless of mode.

How to get
-Go into Nether (I've tried in the overworld in creative)
-Change mode to survival if in creative and mine Warped Nylium with a pickaxe OR blow it up with TNT in any mode
Notes
It is not guaranteed every time, it's seemingly randomish
Only on Warped Nylium
Cannot be placed, Cannot be dropped from hotbar, only in inventory, and it's completely invisible
Not shown in hand ever.
When mining other blocks with it no breaking sound can be heard
Only listed as "Nature" when hovered over, and has no name
Screenshots included