Okay so, I know there's going to be so many people who look at this and go "Don't be stupid" or "Dumb question" or something like that, don't bother commenting at all it's just a waste of your time. For those open minded and still here, prepare for weird thoughts.
I have not seen it specifically stated that Titans necessarily have to resemble humans. Me and my mates have talked about it in length before; they're made from humans which is why they resemble humans, so why not through the power of science has anyone tried combining Titan spinal fluid with animals? Or experimenting on a human and mixing animal DNA? They were used as weapons at one point right? It baffles me that no-one thought to combine them with animals.
The conversation got onto a stranger topic when I and a particular mate in our group pointed out that, since the mass of a Titan comes from some not-well-explained magical space/time portal instead of something like accelerated growth, why are there seeming even limits at all on things like species or the number of limbs? If it was based on science and accelerated growth I would say "Because it's got a human body as its base", but Titans aren't scientific, they're fictional, even within their own story world they're ficitional. They aren't based in logic or science or anything that's bound by the laws of biology and physics, they're born from some mysterious and vague "source of all organic life" hocus pocus and created via magical logic defying space/time malarkey. Having there be some unspoken, unexplained limiter on Titans just seems like an oversight (like there only being nine Titan Shifters when the creators knew full well that every OC creator would think Titan Shifters are awesome and want to make their OC one, so all the OC Titan Shifters end up being "The Mysterious 10th Titan Shifter").
What's to stop a Titan being a giant wolf or a falcon or a lion, or heck even something wackier like a flippin' dragon? It just seems like a lost opportunity to explore an interesting idea.