The Lamassu is an obscure monster from ancient Sumeria, also known as a Shedu. It appears as a winged bull or lion with the head of a human man and is regarded as a protector spirit, as well as being associated with a particular goddess. Its Sumerian origin is probably one of the reasons it's so obscure, as it doesn't originate from Greek/Roman, Egyptian, or Lovecraftian mythos. Muslims adopted the critter as Buraq, the creature that carried Mohammad to Paradise for a one-on-one with God.
In Dungeons & Dragons, Lamassu (usually misspelled as Lammasu) and Shedu are two seperate Good aligned outsiders. Lamassu are winged man-headed lions with the powers of a 7th level Cleric, whilst Shedu are winged man-headed bulls with psionic powers.
In Warhammer Fantasy Battles, one of the mount options for Chaos Dwarf sorcerers is the
Lamassu intentionally-misspelt LammasuTM, which is a magic-eating monster depicted as a winged bull/lion thing with the head of a chaos dwarf of Sumerian fame. Lammasu also appeared as one of the bindable monsters in Storm of Magic.
So what are these guys specifically in Warhammer Fantasy? Well, think of them as the equivalent of one of the Chaos Beasts under Hashut. A Lammasu in a sense is thought to be a rare and interesting mutation of the Great Tauri, which is the other Chaos Beast under Hashut.
In contrast to Hashut's stereotype of his pyrophilia and bull-fetish bestiality, the Lammasu is not consumed by fiery rage but rather, it is a manifestation of sorcerous intelligence. Though not as powerful as its cousin in full-on CQC testosterone fights, the Lammasu is able to employ magic instead, launching fireballs at the enemy, transporting allies on shadowy mounts in a blink or simply letting death creep through enemy ranks. Therefore, think of the Lammasu as the Tzeentch to the Great Taurus' Khorne.
They are also blessed with an uncanny, insidious charisma that can befuddle enemies akin to real-world Sphinxes.
While it takes a great amount of power to summon one, the wizards who invest it will not be disappointed, which is why many Chaos Dwarf sorcerers of a sufficient rank use them as their mounts.
Featured in the Chaos Dwarf DLC for Total War: Warhammer 3.