Indomilisk Rex was an enhanced strain of the primal ultralisk found on Endion. It was kept in a protoss holding pen north of the stasis locks holding Cybros in stasis.
Indomilisk Rex is likely a reference to the Indominus Rex from the 2015 film Jurassic World.
Blizzard Entertainment. StarCraft II: Legacy of the Void. (Activision Blizzard). PC. Mission: Legacy of the Void, Unsealing the Past (in English). 2015-11-10.