Queen Bea has stated that she has used 'Heavy Hollas' in battle. This may mean that there are multiple kinds of Hollas. Heavy Hollas may also be Heavy Units and, if there are other kinds of Hollas, they may or may not be Heavy Units.
Real World References
Hollas being mounts are a pun -- K.C. is described as being on "hollaback", which is a reference to the song "Hollaback Girl", sung by Gwen Stefani.