My mare Jackie is let out in the daytime on my pasture, but I was wondering how 'fat' a healthy horse can be. I will show pictures if needed... I know that if she is too fat she can develop hoof problems, so I don't wanna risk it.
The second picture is really small, but I can see the outline of ribs, and I don't see a crease down her back in the picture from the rear, so I don't think your horse is overweight. If anything, she might be slightly under ideal weight.
If you think your stud maybe pregnant, he is too fat. If you think your gelding may be packing a colt, he is too fat. If your mare is all alone in a pasture, and looks pregnant, you may need to build a better fence... From the small picture I saw, this horse doesn't look terrible. Make sure she is de-wormed.
You should be able to easy feel the ribs, not see them. I couldn't tell anything from the pictures. If her mane is cresty, then that's no good, and the base of her tail is it boney or spungy. Along her back should be flat no crease and no back bone sticking out. Hope this helps
Just to throw a wrench in this whole thing (though I agree with pretty much everything that has been said), it bears pointing out that a horse with something like insulin resistance (not that the mare in question is at all IR, of course) could have fat pads and a cresty neck, but not be overly fat.
For instance, my mare is IR. Even when she's super muscular and very fit, her neck is still somewhat cresty and she still has fat pads behind her shoulders+along her spine. Her ribs are easily felt and she's a perfect weight for her body type, per the vet, but if you based it on fat pads, she's be deathly obese.
For her, I know that due to those fat pads (which disappear when she gets underweight), her ribs disappearing is my sign that she needs to loose. If I can see her ribs when she sneezes/coughs/stretches out at the trot, I know her weight is acceptable. If I stop being able to see a little rib at times and there begins to be a covering of fat over her ribs when I feel for them, I know she needs to cut back a little.
Basically agree with others, tho I think a 4-5 on the Henneke scale is generally ideal and you should be able to easily feel ribs but not *necessarily* see them. Eg. Just because you can see your mare's ribs is no reason to think she's underweight. Likewise people often mistake muscle atrophy on the topline for the horse needing more 'groceries'.
Re IR, I think as with us, horses can indeed afford to get fat, but it's long term overweight without regular 'bad seasons' to use up the fat stores that is the big problem - people feeding their horses up over winter to prevent weightloss for eg. So there's no 'bad season' for them before they go into yet another 'good' one...
Also agree that those pics aren't much help sorry. You can upload pics directly here, and bigger pics showing the whole horse would be good. It seems from those pics that your mare is not much overweight, if at all, but she has got a big belly & also a funky(technical term) lumbar region by the looks. I'd consider getting a bodyworker to check her out.