Well, I'm no dietitian but I'll give this a shot anyways...
Cows & Horses are true vegetarian and produce all the amino acids that they need to build various proteins. Humans are not true vegetarians and must obtain the amino acids that they cannot produce for themselves from their diet.
The rest of this shit is copied from MEDLINEplus...
Proteins are described as essential and nonessential proteins or amino acids. The human body requires approximately 20 amino acids for the synthesis of its proteins.
The body can make only 13 of the amino acids -- these are known as the nonessential amino acids. They are called non-essential because the body can make them and does not need to get them from the diet. There are 9 essential amino acids that are obtained only from food, and not made in the body.
If the protein in a food supplies enough of the essential amino acids, it is called a complete protein. If the protein of a food does not supply all the essential amino acids, it is called an incomplete protein.
All meat and other animal products are sources of complete proteins. These include beef, lamb, pork, poultry, fish, shellfish, eggs, milk, and milk products.
Protein in foods (such as grains, fruits, and vegetables) are either low, incomplete protein or lack one of the essential amino acids. These food sources are considered incomplete proteins.
Plant proteins can be combined to include all of the essential amino acids and form a complete protein. Examples of combined, complete plant proteins are rice and beans, milk and wheat cereal, and corn and beans.
8 amino acids are generally regarded as essential, with two others, histidine and arginine, essential only in children: