#if you're curious about the sheer variety of penises that exist it is a fascinating subject

[frogeyedape]

I had the same question and found this: https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1977.tb04189.x

And this:

Looks like that's what fallow deer prepuce looks like during the rut. The prepuce is the sheath, you can see in the next vid that the erect penis looks more "normal" https://youtu.be/XfzheLKw5Wc

Footage of a black fallow deer recently seen in Baryczy Valley, Poland.

(source)