[@Duthguy]
Like Kazemitsu said, Rattleskull's children are goblins because he reproduced with goblins. If Rattleskull were to find another evolved goblin of the same species as himself, his offspring would instead be of his own species.

I decided on such a thing because that way monster populations aren't interdependent on each other to survive. Orcs can feud with Goblins or Hobgoblins without wiping themselves out, because not all Orcs begin life as Goblins or pass through the Hobgoblin stage. As long as there are enough Orcs to maintain genetic diversity--regardless of what those Orcs started their lives as--they can reproduce with each other to create more Orcs.

Some monsters have specific reproductive capabilities that keep them from being wiped out in a world that is constantly fighting and killing them, or their reproduction involves magic in some fashion so it's a little different, etc. For instance, Goblins are known as rapidly reproducing pests, so they're killed a lot. A male goblin can produce viable offspring with various other species, but in most cases--such as humans, elves, etc--the child will always be a goblin. This adaption means even though a single party of adventurers might wipe out a whole tribe, a single straggler would have enough reproductive options to keep the species from going extinct.

The details have some rather disturbing implications, but hey what's a fantasy world without some lore?