Thorn in plants a process of adaption

Thorn in plants is very common in most of the plants. Very few number of plants develops thorns in their body. Why the plant develops thorns in their body. 

Some plants are found thorns in their fruits, some plants develops thorns bottom of the flower. Some are in hard body. 

You may be found horny animals. Have you ever think that why they develops horns. The cows, the goats and dears are herbivores and found to develops horns. 

The hervivores animals have no sharp teeth and they have flat teeth.