Yahya related to me from Malik from Nafi that Abdullah ibn Umarsaid, "If someone dedicates an animal and then it goes astray or dies,he should provide a substitute, if it was for a vow. If, however, itwas voluntary, then he can either provide a substitute for it or not,as he wishes."
USC-MSA web (English) reference: Book 20, Hadith 158
Arabic reference: Book 20, Hadith 861