Diomedians:
Keshi ponies are pretty much just Earth ponies with diomedian colour schemes.
Shadowbolts are pegasi with bat-like wings.
Tentacle ponies are unicorns with a tentacle instead of a horn, which they can use to grip with or as a whip. The tentacle is around five times the size of a unicorn's horn, since they have to physically grasp whatever it is.
Dark Royalty have two tentacles on their forehead and bat-like wings and are roughly the same size as normal pony royalty, i.e. Celestia-sized.
They tend to have more garish colours in their hair, like green and purple or teal and orange stripes.
Cave ponies(with ability descriptions if you want inspiration for action shots):
Slippy ponies have eight legs, shoot web and are about 60% of the size of a normal pony. They're gifted at weaving, clothesmaking, leatherworking, farming, and mechanics(i.e. traps).
Draltha ponies have a thick lion-like mane around their neck and are about 180% of the size of a normal pony. They're gifted at mining, carpentry, masonry, pottery, and armoursmithing.
Gorlak ponies have two tusks which they can use for telekinesis, and are about 80% of the size of a normal pony. They're gifted at brewing, delicate crafting, and diagnosis.
Underlords are all those mixed together and are four times the size of a normal pony, or 133% the size of normal royalty.
Cave pony hair tends to be reminiscent of rock formations, such as dark red with blue or red spots to represent rubies or sapphires in bauxite, or grey with a dark blue stripe and one of either a light blue, red, green or yellow stripe to represent diamond-bearing kimberlite in gabbro.
Rainbow ponies are just a combo of normal, diomedian and cave ponies, i.e. all the castes are taken from the existing ponies and dumped in.
Honey badger ponies have a honey badger's face, tail and colouration and a pony's body, legs and hooves. Honey badger pegasi have wings, and honey badger berserkers have three horns(two on their head, one on their nose) and tusks. Normal and pegasus are about normal pony size, berserkers are about 40% larger.
If I finally pull my finger out and write those newbie guides, would you mind me using your pics as illustrations? Looking at this big, weird wall of text, I really should prioritise that because some of this stuff is just weird(and the simplified/traditional split). I suppose this is what you get when you mix My Little Pony, Dwarf Fortress, and my specific flavour of crazy.