The modern 1.21.5 spawn egg textures backported to pre-flattening versions (1.8-1.12.2)
The flattening (1.13) was the update that reworked item IDs, including giving each spawn egg a unique model. On versions before this all spawn eggs use the same model, meaning OPTIFINE IS REQUIRED to make use of it's Custom Item Texture feature to enable unique spawn eggs.
Ignore any "Made for a newer version of Minecraft" errors that may appear in the resource pack selector, as long as you are within 1.8-1.12.2 with Optifine the pack will work.
