* Specialization in biology refers to the development of specific structures and functions within an organism.

* Horses are highly specialized mammals. They have evolved for specific tasks like running, grazing, and carrying weight. Their hooves, digestive system, and musculature are all adapted for these functions.

* Paramecia are single-celled organisms with basic structures that perform all life functions.

* Amoeba are also single-celled and have even fewer specialized structures than paramecia.

* Worms are more complex than single-celled organisms but are still much less specialized than horses. They have various body shapes and internal systems, but are not as specifically adapted for a particular niche as horses.

In short, horses are the most complex and highly specialized organism on the list.