Major spoilers about the nature of Reapers in Mass Effect below.

It's said that Reapers are made of and in the image of a specific species that's been harvested. They're supposed to be "nations", collections of specific species to preserve them, hence the Human Reaper at the end of Mass Effect 2. Harbinger was presumably made of some Insectoid species.

However, Reaper Destroyers all look the same. They mostly look like tiny versions of Harbinger with less flourish. It makes sense that Reapers need simple, small attack units, so I suppose it makes sense that Destroyers could be made out of any species.

For reference, this is a Reaper Destroyer as presented in Mass Effect 3:

Is there any evidence throughout the series that suggests how Destroyers are built? They seem contrary to the normal construction of Reapers like the Human Reaper and the failed attempt at a Prothean Reaper.