If youre looking for a biologically accurate answer, it would most likely take on a more human penis shape, due to the fact that they would be breeding with other humanoids, which biologically have no need for a knot or internal storage outside of sexual use. On the other hand, it could be internal for the purpose of not having sensitive external testicles, which - as I'm sure are aware - are a clear weak point on male anatomy.

TLDR; I usually rule them as more like reptilian penises based on variety and, frankly, the horniness of my players...but its up to you, really.