The other brushes appear to be sampled brushes (edit: as created with Edit > Define Brush Preset) whereas the ones you created are based on the »basic« round form.
In any case there is no way to change the icons displayed in the Brush Presets Panel by default.
You could use the list display instead of the thumbnail display so you can see the names without having to hover.
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Towards the end of that thread
some products are presented that might improve Photoshop’s Brush managing experience, for example
how can i remake them and have a custom icon like the other brushes ive downloaded? the define brush preset is greyed out for me..
Those brushes were created with sampled image data and that is reflected in the icon.
When you create a simple round brush that will be reflected in the icon just the same.
the define brush preset is greyed out for me..
Have you checked the Help to see what this feature is supposed to do?