Notice that the speech bubble is referring to "Hanko", not "Hanako". Since no other character has that name, we can infer that the alligator is named "Hanko".
The speaker also makes it clear that Hanko is together here with Hisao. Since we have already determined the identity of the alligator, we can conclude that Hisao is cosplaying a pregnant Hanako while making love with "Hanko" the alligator.
The tags do mention "Hanako", however, so my suspicion is that she is the out-of-frame speaker. Perhaps Hanako is (or was previously) indeed in late pregnancy, making normal sex out of the question, so she allows Hisao to explore his newfound pregnant-transvestite-netorare-bestiality-submission fetish without her.
CID 30903, posted on July 15, 2011: