Thank you for all the replies and suggestions! I think Ive more or less gt it down. Of course, I cannot add every breed (there are 100s of horse breeds and only 12 months). So I sort of had to keep it to horses I most wanted to paint. Ill be working mostly on a complimentary color basis to make the horses pop. Im going to avaid painting people, since I want this to focus on the horses.
Jan - Bay Peruvian Paso/Paso Fino in full traditional festival tack (plates, gold accents, tassels and flowers), performing a dancing gait. With the bay I can work in some red and yellow tones to counter all that blue, and perhaps some blue and red accents in the tack to counteract both colors.
Feb - Two Fjords (classic brown dun) grooming each other. The yellow sheen of their dun coating should go perfectly against the light blue. No tack. I will probably do this as a portrait, and give it some intimacy.
March Red Chestnut Icelandic in a flying pace or tolt. Red goes perfectly with green, but with a chestnut and all that yellow I think I can avoid the Christmas feeling. No tack.
April Buckskin/dun Morgan, either a full body or half body shot. May work this as a profile shot. The yellow and black should work well against the turquoise, and since black is rarely just black, the blue sheen of the black points will work well with the yellow.
May Black Arabian, possibly a portrait or have the horse facing 1/3, trotting. No tack, or perhaps an arabian halter. Again, the blue and red tones in the black should work dramatically with the purple.
June Grulla (high chrome) Mustang rearing, ½ body shot. The darker grulla coloring will really pop against the red and yellow.
July Bay Splash Abaco Barb, a replica of one of the few living members of this breed.
Aug - Palomino Akhal Teke, the one time I would double up on the colors. But here, I should be able to get the golden sheen that akhal tekes have extra emphasis.
Sept Horse Evolution
Oct Chestnut Thoroughbred with a blue ribbon, portrait. Again, the red chestnut against the blue would be lovely, and the blue can be mirrored in the ribbon.
Nov Belgian in Harness, full body shot, maybe 2/3,
Dec Grey Shetland Pony with a red bow. I dislike pink (its really only diluted red. I mean, light blue doesnt get called a separate color. It bugs me) but I may mirror the pink in the gray.
For the cover I may do a grey Carthasian Andalusian performing a Piaffe or Airs above the ground. The white of the grey would be eyecatching for a cover.
Im going to start looking for more references. For horses there is a large database of Deviantart stock. Though if anyone has any pictures, I wont say no. I love pictures! Everyone loves horse pictures! Thank you all again!