Hi @jure. Interesting question!
Plantago is a genus, not a species. It covers around 200 species. So a plantago seed could be any one of approximately 200 plant seeds.
The psyllium seed you describe comes specifically from the species plant plantago ovata, of the genus plantago. I presume you are asking about it because it is a source of dietary fibre or for herbalism.
You could indeed use a non-specific 'plantago seed' because, as far as I am aware, all plantago seeds are edible. However there is a difference between a seed and a husk - in terms of what each achieve.