Cardassians

A Cardassian is a hostile alien species found in Star Trek, they are especially prominent in the Deep Space Nine series but dating back to the Next Generation - Cardassians were introduced so that the Federation could have an enemy species that was capable of being interacted with (unlike the Borg, who were without emotion or individuality).

Gul Dukat is perhaps one of the most well-known individual Cardassians in the series and is also one of the major antagonistic forces of Deep Space Nine (though he was also an ally at times).

Elim Garak is normally not villainous but his Mirror Universe counterpart is a vicious and sadistic Gul in the Cardassian army and one of that universe's many antagonists.