I have another topic that walks through the process but it probably needs some fine tuning and updates.
That topic is here:
To learn the process I recommend opening and editing a current brush rather than just diving in and creating a new brush BECAUSE without using an existing brush as a template we might very likely create a huge brush that won't work very well. So...
Use OpenToonz Browser Room to navigate to the brushes folder where you will be looking for a .PLI file
That file will contain the frames of your brush.
If there is only one frame in the Level it will be a static brush and if there is more than one frame you'll have an animated brush.
I did read somewhere in the OpenToonz documentation where it stated something about only the first frame being used but.... I can't recall the context of what it was referring to.
I do know that we can create/edit animated brushes in this way.
So, open and edit a current brush. Saving it back out via 'Save Level As' and placing it in that same directory with a new name (Don't save over the top of the original brush).
You *might* have to restart OpenToonz to get everything working optimally but I don't think that is required.
Let us know how that works for you!