I have seen Republic of Korea (RoK) Ministry of National Defense use both ‘K9’ and ‘K-9’ when talking about the Thunder.
U.S. Army photo by Private First Class Dasol Choi, 01MAY2016.
U.S. Army (USA) video, 10MAY2016:
USA photo by Sergeant Christopher R. Baker, 15MAR2015.
USA photo by Sergeant Christopher R. Baker, 15MAR2015.
USA photo by Sergeant Christopher R. Baker, 15MAR2015.
USA video, March 2015:
Apparently U.S. artillery guys like what they see in the K9. USA photo by Specialist Steven Hitchcock, 15MAR2015.
USA photo by Specialist Steven Hitchcock, 15MAR2015.
USA photo by Specialist Steven Hitchcock, 15MAR2015.
USA photo by Specialist Steven Hitchcock, 15MAR2015.
RoK Ministry of National Defense promotional/explainer video:
RoK Ministry of National Defense video, K9 crews explain their jobs, includes glimpse of K9 based ammo carrier:
RoK Ministry of National Defense video of K-9s blasting away:
Vehicle I-D: TURKISH TEMPEST, NATO Turkey’s modified K9
Vehicle I-D:
Puolustusvoimat / Finnish Defense Forces photo.
Vehicle I-D: 
KOREAN 대한민국 K1