From Lexicanum

The Laer are non-humanoid aliens. They, along with their planet Laeran, were exterminated by the Emperor's Children (pre-heresy) during the 28th Expedition in a campaign that came to be called the Cleansing of Laeran.

All Laer maintain a snake-like lower body. But the Laer society genetically and chemically modify their bodies to to better suit the task individuals are given. Noted are aerial, land-based and aquatic warrior forms^a.

Canon Conflict

The novel Fulgrim indicates this was before the Emperor's entombment in the Golden Throne, but WD 255(uk) indicates that it took place in 893.M31, long after the Second Founding (021.M31).

Sources

• Fulgrim (Novel) by Graham McNeill p.22 and following.

^a: p. 33

• White Dwarf 255(uk)