Question: Ferdinand had his name since birth when he is being raised at the bull camp. But he is later adopted by Nina and Juan who also name him Ferdinand. How do they know that his name is Ferdinand? He was not wearing any identification tags and he couldn't speak to the humans to tell them this.
Question: Why would the owners of Casa de Toros just give the bulls away at the end? It's a nice thought that the bulls could just run free at Ferdinand's home on the hills, but I'm sure the trainers would have been asking for a pretty big monetary exchange.
Answer: There could be any number of reasons. The owners may have felt that just letting the bulls go was the morally right thing to do without profiting from it.
Answer: It's a very common Hispanic name, so probably just a coincidence that Nina and Juan happened to name him that.