gugalanna (fate)
The Great Bull of Heaven, Gugalanna was a deity in ancient Sumerian religion and was sent to take retribution on Gilgamesh for spurning Ishtar. He ended up being killed by Gilgamesh and Enkidu.
His true form in Fate is a massive skeletal bull as seen in the background of Danbooru post #4883850, which Ishtar (Swimsuit Rider) attempts to revive during the Dead Heat Summer Race event. A younger, smaller version of him accompanies her swimsuit saint graph, seen in the foreground of the same post.
Character Design: PFALZ