Can I use Model function in a plugin?

[eluser]shinokada[/eluser]
I am getting an error with this in plugin.

Code:

$catname = $this->ModelCats->getCategoryNamebyProduct($category_id);

It seems I can't use like $this->Modelname in plugin.

The model is autoloaded.

Can anyone suggest the best way? Do I need to change it to helper or library?