Not a separate class of shares. Almost always, founder shares refer to the magnitude of the grant and the voting-rights agreement attached to it. ("Stock class" is a very specific term.)
This serves my point not yours, the process is often very misunderstood by those being promised the equity, and it's often "converted" in ways that dilutes or outright takes away most of the value by the time the employees ever see financial gains from the equity.
It happened in the largest medtech acquisition in history (at the time) its Public knowledge.