The flea bites are just part of the greying gene and some horses get flea bitten depending on the expression of that gene. It is entirely possible to be more flea bitten on different parts of the body as greying does tend to happen in different areas first. And the color of the flea bites does usually have something to do with the base color of the grey horse.

 

It could be part of the greying but I've also read that the flea-bitten pattern may not be part of the process but related to another gene and it only shows in horses greying. I can't remember where I read that though.

Also they do change colour at different rates over the years and areas of the coat.

It could be a very large 'blood shoulder/mark'. When I was young, a Highland, either Jock or Stookie, anyway, one of them, had the dark grey patch down his lower neck and shoulder where his coat wasn't greying out as fast as the rest of him. It looked like someone had spilled paint on him.

 

Could also be a grey pinto. Unlikely if it's symmetrical on both sides and if there's a gradient of fleabitten rather than a delineation, but not impossible depending how he looks!

 

There's a new pony at our barn that is a fleabitten grey. However, looking at his coloration, you'd think he was two different horses. From the shoulders forward he is VERY fleabitten. From the shoulders back, he's pure white.

He certainly sounds like a handsome fellow if nothing else.

This got me to thinking, what do "fleabitten" marks come from?

They're part of the graying process. I've heard that heterozygous grays are more likely to have them, as they supposedly "gray out" slower than homozygous grays. DNA evidence has yet to support that theory though... as far as I'm aware anyway.

Arabians in particular are well known for producing flea-bit grays. Of note, one of the two flea-bit grays that I knew was an Arabian mare. The other was a Quarter horse gelding.

Do they bear any relationship to the horse's color before it went grey?

Yes, the flea-bits are supposed to be the base color. IE: What color the horse is genetically, rather than what it appears to be. Remember, Gray is a color modifier gene - It's not actually a color in of itself.

Flea-bits do seem to be most common in chestnuts/sorrels for whatever reason. I have heard that bays can get them too though. Both of the flea-bit grays that I knew were originally chestnuts, so their flea-bits were red.

Is it just random chance that this horse is basically two different colors, one in the back and one in the front?

Have you asked the owner of the flea-bit pony what's going on with their coloration? They (hopefully) would know what's going on their own animal's coloring and explain it to you.

Regardless, the pony's unusual looks could be caused by almost anything.

Some flea-bits have what are called "Blood Marks". Heavily pigmented, irregularly shaped, almost patch like sections on the horses' body. They often occur on or around the shoulders, hence a common alternative name for them- "Bloody Shoulders". However, they can and do appear in other areas as well.

Typically associated with Arabians, who come with a lovely legend that tells of how they first acquired them to boot. This is one variant of the tale- The Legend of the Bloody Shouldered Mare

The pony may have been born a pinto. The flea-bit sections of his body are merely where his original color was located, the surrounding white areas are his white patterning. If you ever see his owner giving him a bath, pay close attention- You'll be able to tell his colored patches and white patterning apart just going off of the skin color! White patterning causes pink skin, non white patterned areas -Yes, including grays- will have dark skin. Such horses are commonly referred to as ghost pintos. The fur may lie, but the skin underneath always tells the truth.

Then there's crazy stuff like Somatic Mutations and Chimerism...

 

Yes, the flea-bits are supposed to be the base color. IE: What color the horse is genetically, rather than what it appears to be. Remember, Gray is a color modifier gene - It's not actually a color in of itself.

As many black Percheron with red flea bites as I've seen I'd say there's something else at play. I have my suspicions but that's all they are.

 

"Flea bites" get thicker or more abundant with age. Greys, at least with Arabians, tend to grey out fairly slowly turning pure white and then slowly the flea bites start appearing and getting more apparent as they get older. I can't find the picture I'm looking for, I'll keep looking. When I met this mare, AV Marista (Ariston X Bint Marwa) she was white with very few flea bites, by the time she died, she was almost liver chestnut with silver mane and tail. Really a stunning mare, regardless of color, but she was an outstanding example of a flea bitten horse.

 

Yes, we have observed what @Dreamcatcher Arabians is talking about in Harley. He was born chestnut, then greyed out, with flea bites, which are now getting bigger as he ages. They're also much more visible in the summer. His winter coat tends to be whiter so the flea bites disappear a little. He looks like a different horse. It's really fascinating!

And then there's Rusty's weird coat patterns. Under his leopard appaloosa spots, he has some black mottling on his chest and neck, all the way to his face. So there are two colour patterns going on - the mottling is more black whereas his typical Appy spots are chestnut. As a baby, he was more chestnut.

 

By the time Marista passed, she was in her 30's, it didn't matter if it was summer or winter, she had more liver flea bites visible than she did white. Her mane and tail were really outstanding against all those flea bites. I hope I can find her pic, she was a stunner and one of the sweetest mares I've ever met. Made beautiful babies too.

 

A couple of horses at my barn are flea-bitten greys.

I hate that terminology. I think of them as white with freckles and they are used in wedding and engagement shoots so clearly too gorgeous to be called “flea-bitten.”

Their spots are liver/chestnut-ish so interesting to wonder if they were fully those colours at birth and then greyed out - is that always how this coat colour develops? I think the two horses at my barn are lighter in winter too.

 

From my understanding a heterozygous gray is going to have flea bites while a homozygous will not but I've not researched it. Base color isn't always the same as flea bite color. I will say those with black bites were black to begin and seem to be rare. Most are red no matter the base, even if black. Even with flea bites they tend to go through an all white stage and concentration of colored spots varies.