No, he's earnestly saying what he's saying. Unfortunately, you don't have the full discussion online. The exact details are that it was argued that slaves are merely property, and thus their complaint/petition cannot be presented. Adams responded, by saying that the argument of property has no bearing on the matter. Slave, horse, dog, whatever, hear the complaint, for nobody (non-slave, slave, animal, etc.) has the right to not be heard.
The dictation presumably taken by others, indicated he was making a mockery of several congressmen and representatives from various states with regards to petitions of slaves made to the state in which they were bound.
Don't you get a metaphor when you see one? You're concluding from his remark that he believed in animal rights, when he never said any such thing. If you have the complete transcript to prove otherwise, then supply it. Every other reference to it mentions him mocking his colleagues, not making a case for animal rights.