The Beast Marked By Love is a manga-omake included with Rai no Sho. It is set during the events of the Blank Period after the events of The Last: Naruto the Movie. The omake is unique as it is through Akamaru's point of view on his thoughts on master/partner Kiba Inuzuka.
Summary
As Akamaru and Kiba go for a walk, Akamaru beings to notice how Shikamaru Nara, Chōji Akimichi, and Sai have formed romantic relationships with Temari, Karui, and Ino Yamanaka respectively, which he scoffs at how humans enter estrous later in life and his pride that his master Kiba hasn't fallen for such "distractions", that is until Kiba immediately notices a young kunoichi walking her dog, Shōshi, in a park an attempts to flirt with her albeit unsuccessfully. Kiba orders Akamaru to "mark their territory" all over the park saying that all the girls that dumped him will not have another shot with him once he's the Seventh Hokage. However, he immediately becomes happy when he two other girls remarking that they see the future Seventh (thinking they are talking about him), only to become distraught when he realises they are gushing over his "rival" Naruto Uzumaki and Hinata Hyūga much to his annoyance and depression that he doesn't have a girlfriend yet. Akamaru however, is still quite pleased with Kiba and feels the girls know nothing and has full confidence that Kiba is going to be the Seventh Hokage.
Later, Akamaru becomes suspicious when Kiba orders him to "stay" as he goes out alone. Smelling that he is being deceitful, Akamaru follows him only to discover that Kiba is privately meeting a young girl much to his annoyance. Walking home alone, Akamaru admits to himself that Naruto is far more powerful than Kiba and is better suited to be the Seventh and that Kiba isn't being "focused", but it's not he's massively unpopular with girls, which upsets Akamaru that Kiba's recent focus on trying to get a girlfriend and not being a better ninja is getting in the way of his goals.
Back at home, Kiba tries to give Akamaru food, but he refuses due to his anger and when the girl from before walks in, he becomes hostile. Kiba holds Akamaru down as she reveals herself to be a vet and gives Akamaru his routine immunisations which he hates, hence why Kiba's secrecy and the sleeping powder he put in his doggy dish. This revelation relieves Akamaru, though he's admittedly still angry about the shot.
During another walk, Akamaru is happy Kiba will always be with him because of his massive unpopularity with girls. However, Kiba suddenly stops dead in his tracks as he sees Tamaki for the first time despite her having a cat. Akamaru is horrified when he realises the two have a strong mutual attraction to each other. With this, he does his "duty" to "protect" his master by trying to fruitlessly pull him away from Tamaki, much to Kiba's protest.