When it comes to the therapeutic applications of "chimeras," or human-animal hybrid embryos, I'm inclined to agree with Lindsay Beyestein's sentiment that making Leon Kass's head explode is a nice bonus on top of the medical promise of the research. The objections advanced in this article by various bioethicists strike me as not simply wrong, but obviously, stupidly wrong. Anyone who thinks potentially lifesaving research involving these embryonic hybrids should be foresworn because "It would deny that there is something distinctive and valuable about human beings that ought to be honored and protected," as though what's valuable about humans is the distinctiveness of the genome, is a special kind of moral imbecile. Thomas Paine's cutting comment in a different context is apt: "He pities the plumage, but forgets the dying bird."
That said, there are interesting ethical questions when it comes to the possibility—not yet seriously broached by any researchers, but likely to come up eventually—of bringing a "chimera" to term rather than simply destroying the embryo after a few days for research purposes. While I don't share some people's instinctive horror at the prospect, there's intuitively something wrong with deliberately bringing someone (assume one could create a chimera with something approximating human consciousness) into the world condemned to be a kind of ultimate outsider: The lone member of their species. That's even assuming one knew it could be gotten right, so to speak, and that the hybrid creature wouldn't just die of organ failure or some such thing at a young age.
But as Derek Parfit famously observed, when we attempt to consider cases like these by asking whose rights are violated or who is wronged, things get tricky. You initially want to say: "Well, the hybrid child, of course." Yet it's not as though the hybrid status is some kind of extra burden inflicted on some determinate, existing person. That is, we cannot say (as in the case of ordinary harms): "You made the child a hybrid zebra (or whatever), whereas he clearly would have been better off, had an easier go of it, as an ordinary human." That's because "he" would not have been at all: Any child, however genetically similar otherwise, with a fully human genome would have been a different person. Unless the child's life is literally worse than non-existence, then, it's hard to cash out the intuition that there's something wrong with doing this in the language of "harms" or "making the child worse off." A straight utilitarian analysis might seem to give the answer that comports with our intuitions in the instance—if a distinct but fully human child could have been created instead of the chimera, that would've been a state of affairs with greater net (and average) utility—but that raises a whole host of other thorny problems, which Parfit explores deftly in his seminal Reasons and Persons.