For anyone else searching how to have custom spawn egg textures (in Optifine), I've cracked it for 1.12.
The new entity tags are a lot simpler than IDs used to be! I was also missing a couple of bits that are required now.
Create one .properties file per texture, in assets\minecraft\mcpatcher\cit .
Put this in it:
type=item items=spawn_egg nbt.EntityTag.id=minecraft:blaze texture=spawn-blaze.png
First line is mandatory.
Second line is "Name" from the infobox on the spawn egg wiki page.
Fourth line: whatever you've called your replacement texture file in assets\minecraft\mcpatcher\cit .
Tags are case sensitive.