The "Bull of Heaven" was a Sumerian beast oftentimes depicted in cylinder seals. In some traditions, the bull was killed by Gilgamesh.
Background
The Bull of Heaven was a mythical beast prominent in Sumerian mythological stories involving Gilgamesh. According to Jeremy Black, the Bull of Heaven was involved with conflicts with the deities Ishtar and Gilgamesh.
"The Bull of Heaven was a mythical beast demanded
by Ištar (Inana) from her father Anu
(An) so as to destroy the city of Uruk when her
amorous advances toward Gilgameš were repudiated
by the hero. The bull caused widespread
destruction but was eventually killed by
Gilgameš with the assistance of Enkidu. As a
taunt, Gilgameš dedicated the animal's horns
to his personal god Lugalbanda. The story is
told both in the Sumerian poem 'Gilgameš and
the Bull of Heaven' and in tablet VI of the
Babylonian Epic of Gilgameš."
The Bull of Heaven is also prominent in iconography from Mesopotamia. However, the bull is a general motif, making its presence and identification confusing in some instances.