What is an animal or plant taxon (e.g. family, genus, or species) that has a good fossil record of evolutionary development?

The horse is an animal we have a relatively good fossil record for, and even if there are gaps in it, it is complete enough that we can follow, explain and see why it has changed over the (millions of) years.

(ignore the writing a the bottom)

The first horses, were small, diminutive herbivores which are thought to haven inhabited swampy and dense woodland, They were well adapted to this sort of terrain as is evident with their feet-like appendages, which would spread out their weight on the soft ground and prevent them from sinking, and move quickly between the trees.

The next 2 horses are the jump from marsh land to grassland, around about that time, many of the large major forests, and marshy land dried up, throughout Africa and much of the world began to shrink in size.
Due to this, there was often large expanses of grassland and savannah developing and a lack of cover, and also plenty of predators waiting in situ for the unwary, early horse.

Survival of the fittest would have led to those horses with the better feet, that being a 'hoof' which allowed them to cross the open ground quicker, as well as acting as a defensive weapon, than a multi-toed foot.
This would have led to them surviving and passing on these characteristics to their offspring, and those without, being eaten.

The 4th horse is the final horse, which is adapted perfectly for a life of the grassland, which can be seen also as its teeth, like the 3rd horse, are very molar heavy, for chewing on the hard grasses to get the nutrition they would need.
These horses are also very similar to wild horses, and those we have to day, so they're not far off our modern horses today.
Though this would have been another million years to have the sort of horses, which are like those we use in labour, though there are some wild horses with very similar characteristics still too.

Hope this helps.

-Charlie