I imagine the most common question in discussions of creating animal-human hybrids is "why do it?" Julian Savulescu wrote an article for the American Journal of Bioethics addressing this question. He assents that there may be questionable motives - "commercial exploitation of 'freaks'; artistic motivation...or curiosity, just to see what it is like" (22). But he also discusses reasons that are more difficult to dismiss.
Medical purposes: Studying oncogenesis, as source of stem cells, or combining our genes with those of a species resistant to certain diseases
Delay aging or prolong human life: Could we incorporate turtle genetic sequences into our own to reduce telomere degradation?
Enhance human capabilities: Incorporate an elephant's memory genes, an owl's night vision, or a bat's ability to navigate in the dark.
Savulescu argues against those who use "moral confusion" as a reason not to explore the creation of human-animal chimeras and hybrids. He likens their rhetoric to that around interracial marriages, and transgressing species boundaries to transgressing race boundaries. That is not an argument I'm ready to get behind. The distance between a black human and a white human is far different than the distance between a human and a non-human animal. (Plus, who is the animal-parallel in his comparison?) However, I agree that moral confusion is not a reason in itself to halt hybrid research.
He does raise very important questions about species membership, though. He writes, "Why does having a certain genetic structure, for example 46 chromosomes, morally matter?" (23). He argues that "any attempt to base moral status on biology is fundamentally flowed...We share about 98.5% of our DNA with chimps, and the differences between us and chimps might not be due entirely to the 1.5% of DNA that is different but rather the regulation of the genes that we have in common" (23).
Savulescu feels it is a mistake to engage in "biology worship." I imagine one's thoughts on altering our biology may be heavily influenced by religion. If one sees us as a result of evolution, an amalgamation of chance mutations, altering our genetic makeup is permissible. If one sees us as created in God's image, altering this image would be unthinkable. Savulescu, for one, argues that "our biology is not sacrosanct. We should change it to make our lives longer and better" (24).
Savulescu, Julian. "Human-Animal Transgenesis and Chimeras Might Be an Expression of Our Humanity." The American Journal of Bioethics 3:3 (2003).