I haven't seen it yet and I've been looking. But then again, I've had 40 years to work up to it.
I can remember when I was starting out and encountered a knot so big my body just rejected it. It was already inside me so the rejection process was quite painful but not permanently damaging. This rejection process is a reflex and one of the first things a newbie must learn to control.
Rating the size of a knot (or flare) is a very slippery thing. The level of excitement of the owner of said knot (or flare) can alter its ultimate size and the moment of greatest excitement is not the easiest time to measure anything. Nor are natural organs given to symmetrical sizes. I have known dogs with tapered knots and knots with double spherical shapes.
Your perception can change also with different textures and lubricity. The energy with which said organ is being manipulated can change your interpretation of the act too.
But through all that, the biggest challenge for most humans seeking to allow animals entry to their body is the length of the animal's organ. Animals, having 4 feet, tend to live their life with their spinal cord in a horizontal mode. Humans, having only 2 feet, have to keep theirs mostly vertical.
This variation in directionality tends to make for a basic difference in internal organ structure. Gravity compacts the human organs toward their pelvis in contrast to the animals where gravity pulls the organs away from their pelvis. This means that animals have longer vaginal tracts and matching longer penis structures.
The extreme asymmetric structure of human bodies also makes matching them to animal bodies problematic. Either your hips will be too high or your body will lack the length that the animal needs to conveniently handle you.
But given sufficient incentive, a lot of adaptation is possible and few incentives compare to a good orgasm. Any body part can be stretched and/or strengthened. Some parts, like the colon, can even be moved.
So it is possible for a human to counterfeit many animals including any dog or up to a medium size horse.
Bernard