Mushroom horses eat fodder like normal horses, but use straw for the points to eventually get the prize. Having straw in stock is easy to do and common knowledge with a little practice that growing something is cheaper than buying it...

Fire horses need around 10 pieces of wood or less a day. To be honest, this one you should always have in stock if you own an EC to fix boxes and workshops when needed.

To answer your question, Greyfells need Pandora's boxes and Black orchids to keep bringing them back.

Fire horses require 1 wood daily, it's basically the same as a carrot for them. I believe 1 wood costs 10 equus? But they can make this back up pretty easily via missions and comps. If you have a forest EC and board them there they make up what they use and then some.

Plant divines use droppings, but you can choose the amount to give. Max is 200, min is 10. The skill points they get vary depending on what season it is, with winter being the least and summer I believe giving the most, so if you want to get the most for your droppings you can feed them in summer only, or if you have lots of droppings and don't care you can do it year-round.

Ones that have a significant and annoying time investiture are Dragons (main one, takes forever to click through their little battle thing. You can turn off the animation which makes it go faster, though). Nordics and Extraterrestrials (not as bad, but you do have to click around a little to get the right ride/time length) and Chimera (they refuse care during certain seasons even after taming, which is obnoxious and means it can take a while to care for them if they're being really stubborn. I only have one Chimera and I hate him )

Stellars and the Sherlock Holmes ones also take a few clicks but personally I don't find the game takes that long so I've never had as much of a problem with those.

Fairy Tales and Art History ones don't take long to care for, but they're insanely stingy and it takes months for them to drop, so a lot of people find them to be a ripoff just because of that.

Those are the only ones I know. I don't have every series, but most you just have to click once per day (or never, in the case of Love Divines. Best series imo )

I forgot the Greyfells. Yeah, DON'T get a Greyfell. I've heard you can get their perk without Black Orchids as previously mentioned (I don't own one though, so wouldn't know for sure), but they do 100% need Pandora's Boxes and only give 10 passes back (out of a 95-pass sink). Only worth it if you have a ton of Pandora's Boxes in stock, otherwise don't bother.

I only have one Fire horse so I may have been wrong about how much wood they take, but it's not much, I know that at least.

Greyfells give 20p and 20,000e, and you can get their reward easily by just leaving them alone, as their health deteriorates daily regardless of if you work them or not. Having a couple of Greyfells is okay, though I couldn't imagine trying to balance all 12 of the series. You can use BMI trades for Pandora's Boxes instead of buying them every time.

I think you need a divine that gives them like freya

honestly pandoras aren't even that hard to get from events and luck items as well. I stopped caring for my pandoras droppers and I have 51 of them in inventory right now. Theyre literally one of the easier prizes to get from events.