Fire's Clay Spawn is a mod I made to spawn clay underground like an ore. The clay can be set to spawn at the same rate and layers as Coal, Iron(default), Gold, Diamond, Redstone, Lapis, and Emerald. You can also manually override the maximum layer it generates at, the minimum layer it generates at, and the ore vein size. It also supports generation in styles similar to ores from Forestry, Brain Stone Mod, and Draconic Evolution. I plan to add support for generating in styles similar to ores from more mods as well. All of these settings can be applied to Hardened Clay generation as well. Also, the mod can be set to generate Stained Hardened Clay, adding a bit of color to your mines.
License: GNU LGPLv3
Retrogen can be achieved in Minecraft 1.8.9-1.10.2 using SimpleRetrogen by cpw and in Minecraft 1.12.2 using Easy Retrogen by KitsuneAlex. Just add "the_fireplace.clayspawn.worldgen.WorldGeneratorClay" and/or "the_fireplace.clayspawn.worldgen.WorldGenGlazedTerracotta" to the retrogen mod's config.
For those of you saying this is a duplicate of Pam's Clay Spawn:
First of all, this mod is a lot more configurable than Pam's Clay Spawn. On mine, you can manually set the highest layer, lowest layer, vein size, and vein count, or input the name of a vanilla ore and some other modded ores to get the vein size and/or highest layer and/or lowest layer and/or vein count from. And you can generate Stained Terracotta and Glazed Terracotta.