But, you are in Spain, laws and attitudes for suing may be different. Yes, there are these contracts in the US, but doesn’t mean a person can’t successfully sue. Besides, why would someone whose business is boarding horses put that business in a precarious position bc of 2 aggressive roosters?
I’m confident that the cost of materials and time to build a nice, roomy pen for the roosters is less than the cost of a lawyer drawing up this kind of contract or the loss of potential income due to client not being willing to sign such a waiver.