Despite her status as a deferred feature, Mother Hydra is fully implemented in SLASH'EM, and can in fact be summoned in wizard mode. One could even (fleetingly) see her in a normal game by casting stone to flesh on a wished-for statue of her, producing a doppelganger that would briefly mimic her shape. In this regard, she is similar to Cthulhu.
Compared to Cthulhu, Mother Hydra is in some ways more dangerous: her attacks will cut right through even the thickest of armor, dealing 100+ damage per turn to a character in a full ascension kit. On the other hand, she lacks Cthulhu's spellcasting and resurrection abilities, and in fact has no resistances other than cold or poison. A cockatrice corpse or egg will make short work of her.