If we feed animals one food they take their needs and become strong , whether they eat grass or meat. Why do humans need more than one kind of food (RDA) for energy? and if it has a deficient element of the food deficiency symptoms and malnutrition happen?

The requirement of ESSENTIAL NUTRIENTS  ( Not synthesized by body) are different for different species of animals and human beings. So human needs diversified foods not only due to the reason of availability of Essential nutrients but also for change in taste and aesthetic senses. Consumption of same foods for longer periods of time in human being will not ensure all nutrients deposits in the human body So we talk about balanced diets to avoid different ailments.

To describe these things concept of food pyramid evolved. The USDA food pyramid was created in 1992 and divided into six horizontal sections containing depictions of foods from each section's food group. It was updated in 2005 with colorful vertical wedges replacing the horizontal sections and renamed MyPyramid. MyPyramid was often displayed with the food images absent, creating a more abstract design. In an effort to restructure food nutrition guidelines, the USDA rolled out its new MyPlate program in June 2011. My Plate is divided into four slightly different sized quadrants, with fruits and vegetables taking up half the space, and grains and protein making up the other half. The vegetables and grains portions are the largest of the four. The World Health Organization, in conjunction with the Food and Agriculture Organization, published guidelines that can effectively be represented in a food pyramid relating to objectives to prevent obesity, chronic diseases and dental caries based on meta-analysis  though they represent it as a table rather than a "pyramid". The structure is similar in some respects to the USDA food pyramid, but there are clear distinctions between types of fats, and a more dramatic distinction where carbohydrates are split on the basis of free sugars versus sugars in their natural form. More details can be downloaded from following link:

http://www.cnpp.usda.gov/sites/default/files/archived_projects/FGPPamphlet.pdf