Diagrams & Pics with explanations
For the sake of clarity this diagram shows the lateral cartilages smaller than the digital cushion in a lateral view. In reality the LC's are bigger from this angle, but you wouldn't see the DC well if I drew the LC bigger!
A healthy hoof has the 3rd phalanx(pedal bone) well balanced on the distal(ground) surface with the heel raised around 3-5 degrees. P3 is aligned with the rest of the limb with the dorsal wall and lower pastern aligned. The toe and heel are both short, the sole is thick and may be quite concave(depending on horse and work surface). The back of the foot(heels, digital cushions, lateral cartilages) are firm and well developed.
Horses commonly develop a range of hoof distortion, due to a number of causes. Diet and environment are huge factors, which can damage lamellar connections, allow hooves to overgrow, increase susceptability to infections... fail to provide what is needed to develop a strong, functional hoof in the first place. Mechanical factors, such as too much pressure being placed on the walls and too little support under the foot can lead to 'distal descent' as the internal structures 'sink' within the capsule and the sole thins.
Leverage forces on long walls, particularly at the toe cause stretching and separation of the laminae from the ground surface and resulting flares, cracks and infection. Ground parallel or lower angles of the back of P3 cause a 'broken back' pastern angle, putting a lot of stress on both the extensor process(top/front of pedal bone) region and the navicular region.
Alternately, high heels cause 'rotation' of P3 the other way, with the toe pointing into the ground. A horse may have 'clubby' feet for a variety of reasons. One reason is failure of development of the back of the foot, which causes him to be uncomfortable impacting on his heels. Landing toe first naturally wears the toes shorter and allows heels to grow high. Combined with breakdown of lamellar connection due to laminitis &/or failure to manage the toes from flaring with the added pressure, the hoof walls can separate completely. The sole can be flat or even bulging at the tip of P3. In severe cases P3 can even penetrate the sole and in chronic cases the tip of P3 is lost or 'remodelled'.
Below show some examples of 'good' hooves I look after. Not all perfect, but strong and sound on all but the roughest terrain - which is all these particular horses are booted for. An hour or so bare on gravel roads & trails is nothing to these horses.
As an aside, you might notice that a few of those strong, healthy hooves are white. There is no reason for an unpigmented wall to be any weaker than a dark coloured one. Studies have shown there is no biochemical difference aside from pigment, and there is no strength inherent to pigment. You might also notice, immediately after trimming, as per the last pic above, you can see that ALL hooves are white, unpigmented, inside the external layer!
Below; Some unfortunately rather common examples. A. shows lateral imbalance ~ higher right side causing flared left side. B. shows foundered pony part way through recovery. Note still high, imbalanced & contracted heels. C. shows a horse featuring in the 'before & after' pics below, pre Natural Hoofcare. Over at the knee & prone to tripping and lameness from abscesses. D. & E. show similar feet, bottom one just deshod, with very long, underslung and contracted heels and very long toes.
Here are some comparison pics of the same horse's feet. Sometimes befores & afters are close, some are many months apart. The afters aren't meant to portray perfect feet - sometimes they show 'works' still very much 'in progress', sometimes they show great feet. But they are meant to be egs of what can be achieved to improve even terrible feet.
Below; the first trim of this poor fella included a saw! Took a few trims to find the foot under there! Bottom right is the most recent pic. Obviously we're far from there yet...
Below are a couple of 'before & after' examples. The 'befores' are on top. The first pic shows a neglected pony & the 'after' pictures are immediately after the first trim.
The 'after' egs below are around 9 months after. Notice this horse is one who's eg. is now above in my 'good' examples!
And this one - recognise the befores above? Less than a year later, he was a surefooted, completely sound horse, able to go for miles on rough ground. He's still rather flat soled here, but not long after the most recent pics, he quite suddenly gained a lot more concavity.