Thrush *usually* comes up in a horse that is in a wet or muddy pasture; however, it doesn't have to be muddy, could be just constant wet grass - but it can come up in a dry place, too, if you have a horse with health issues and/or a bad trim; it can show up in a horse that just has a poor trim and doesn't get a cleaning. Also, as I stated, it turns up in horses that have other health issues more often (like metabolic problems). It is fungal. It is mostly black like tar and stinks - really different than the horse poop smell - once you smell it, you will know something is off. A lot of times it starts deep in the collateral groove or the sulcus. When and if you see it, I suggest cleaning really well and squirting some ACV on the hoof (it can also show up as a soft, degenerating frog and may not be black and sticky - it depends on the strain of fungus - thrush is just a general term for a fungal infection, but there are A LOT of different fungal strains). ACV is a good treatment for thrush, doesn't damage live tissue, and can nip thrush in the bud before it becomes a real problem. Get your farrier to come out and give the horse a good, balanced trim, and if your horse is in mud or just wet pasture a lot, it is a good idea to clean daily and you can spray ACV on the hoof. It also helps NOT to have a sugary diet or be overweight.