IQM
IQM is a binary-skeletal model format developed by Lee Salzman (aka eihrul) as a "replacement for the aging MD5 and SMD skeletal-animation formats used in Quake-derived and Quake-like 3D engines".
Using in GZDoom
An IQM can be exported from a tool such as 
Noesis (software) or Blender (software). These models can contain a mesh, or a mesh and animation data.
Because of how IQM works, an IQM containing only animation data is unsupported as it needs mesh data to export joints, which are necessary for calculating bone positions. As such the best way to export a series of models that use the same armature would be to export the animation model as an IQM with a single triangle and use it as the animation clip for model index 0, and have the parts that are meant to be drawn as IQMs without any animation data in them as the models.
