Terrorist groups - home based:
al-Qa'ida in the Arabian Peninsula (AQAP): aim(s): overthrow the Yemen Government and Huthi forces and, ultimately, establish an Islamic caliphate; eradicate US and Western influence and presence from Yemen and the rest of the Arabian Peninsula
area(s) of operation: a core al-Qa'ida affiliate; most active in southern, eastern, and central Yemen (2018)
Islamic State of Iraq and ash-Sham-Yemen (ISIS-Yemen): aim(s): replace the Yemen Government and Huthi forces with an Islamic state and implement ISIS's strict interpretation of sharia
area(s) of operation: operational primarily in south and central Yemen, where operatives conduct attacks against Huthi forces, Shia Muslims, and government facilities and personnel (2018)

Terrorist groups - foreign based:
Islamic Revolutionary Guard Corps -- Qods Force (IRGC-QF):

aim(s):  spread Iran’s influence and counter Saudi and UAE by supporting Yemen’s Houthi government through funding, training, logistical support, and weapons, to include ballistic missile and unmanned aerial vehicle (UAV) technology
area(s) of operations:  Yemen, including the Red Sea (2019)