Thick Body: Most of the lower-leveled Cogs such as Flunkies have this body structure.
Thin Body: Many Tier 4-6 Cogs have this body type. Examples include Spin Doctors, Mover & Shakers, and Loan Sharks.
Short Muscular: Specifically for Cogs with a broad chested structure, but lower ranks than top Tier Cogs, like Name Droppers and Two-Faces.
Tall Muscular: Majority of Tier 7-8 Cogs, the highest ranking and highest-leveled ones, have this body type. Examples include Minglers, Mr. Hollywoods, and Big Wigs.
As said by Doctor Surlee, the Skelecogs invading during Doomsday were a result of the fighting Toons destroying Cogs faster than they could be produced. This was also the first time Skelecogs invaded Toontown.
There have been several Skelecog mega-invasions in Toontown Rewritten.
The first Skelecog mega-invasion happened on November 21, 2014 and lasted throughout that weekend.
Subsequent mega-invasions occurred in the years following.
The most recent Skelecog mega-invasion occurred on June 12, 2017 and lasted two days.