Nimrod has a new origin based on House of X # 3
Nimrod gets a bit of upgrade in terms of origin as revealed in the data sheet from House of X # 3 by Jonathan Hickman and Pepe Larraz.
According the new background that Hickman, Nimrod isnt just an off-shoot of the Sentinels. Its actually an evolution from the Sentinels.
In the data sheet, its revealed that Nimrod is the final stage of evolution for various mutant killing machines called the Sentinels.
Its also revealed that they need a number of components before they can actually be built, namely the Mother Mold or rather A Mother Mold.
The tl;dr version of this is that a mother mold is a machine capable of making master molds, who can then make Sentinels. Mother Molds are also the only ones capable of making Nimrods.