The Kantian approach is going to claim that we should only classify rational creatures as moral patients because only those creatures deserve to be treated rationally, and they deserve that because of their own reason.

Funny when Kantians seem to almost brag about this shit when it's actually a reductio

In a sense for Kant, reason dictates that reason must be reciprocated. So I only need to treat rational creatures rationally. I do not need to treat tables rationally, for they are not rational, and thus they can make no claims about deserving to be treated rationally. For Kant, tables and chickens are alike in this respect. Neither are rational creatures and thus neither deserve to be treated rationally. Since, for Kant, reason is the absolute core of all moral decision making, this then implies that we do not have moral duties to nonrational creatures, like chickens. If Kant had built his moral apparatus on pleasure and pain and then said that we only need to give moral consideration to rational creatures, sure that would be very odd. But for Kant, morality is the practical application of pure reason alone, and thus it should apply to all reason-able things, and to only those things.