anti-royalist
adjective
an·ti-roy·al·ist 
ˌan-tē-ˈrȯi-ə-list,
ˌan-tī- variants or anti-royal
ˌan-tē-ˈrȯi(-ə)l
ˌan-tī-
: opposing or hostile toward monarchical government
anti-royal forces
Ten people were arrested at Charing Cross railway station for carrying anti-royalist placards …
Tracy McVeigh
Tracy McVeigh anti-royalist noun
On Jan. 31, 1606, anti-royalist Guy Fawkes was executed for his part in the Gunpowder Plot to blow up England's houses of Parliament. The Chicago Tribune
The Chicago Tribune 
Word History
First Known Use
1782, in the meaning defined above