Saint Christopher, before the Catholic Church started sanitizing him in the late medieval period. In the early legends of him, he was a 9' tall dog-headed man who lived for 200+ years before being martyred after losing a debate to the Emperor Decius. (Because he couldn't speak but could only bark.)
I've always been baffled by D&D's reluctance to include fairly standard medieval legendary creatures like the cynocephali (dog-headed men) who were a staple of Roman and medieval encyclopedias.
As for Loki, I have a different favourite story than Teflon Billy does. I like the one where Odin, Thor and Loki go to Utgaard to fight Loki. I hate it when people clean up Norse mythology to make it sound too rational.