In a fanfiction where Spike is betrayed, it might involve Twilight Sparkle turning her back on him. Perhaps Twilight becomes so obsessed with her studies or a new magical discovery that she ignores Spike's needs and feelings. Spike, who has always been loyal to Twilight, would be deeply hurt by this perceived betrayal.
In some fanfictions, Twilight might be betrayed by a new pony in town. This new pony befriends Twilight, gains her trust, and then uses that trust to get access to the library's restricted section. Once inside, they steal a powerful magical artifact, and Twilight is left to take the blame for the security breach, feeling deeply betrayed.
A possible plot could be centered around Spike's loyalty to Twilight. He discovers a secret that Twilight doesn't want anyone to know. Instead of thanking Spike for keeping it, Twilight blames him for finding out in the first place. This betrayal from Twilight, the one he admires the most, shatters Spike's world. He then has to decide whether to stay loyal or move on.
He could be betrayed by a false friend in the DxD world who spreads lies about him. For instance, telling others that Naruto has a hidden agenda to take over their realm.
In a more complex scenario, Naruto could be influenced by an outside force. Let's say an enemy of Konoha uses mind - control or some form of manipulation on Naruto. They make him believe that the ANBU is the enemy of the village and that he needs to take them down. As he betrays the ANBU, he slowly starts to realize that he's been used, but by then he's already in too deep and has to find a way to fix the situation and regain the trust of his friends and the village.
In this fanfiction scenario, Naruto's betrayal by Akeno could be due to a misunderstanding of their goals. Maybe Akeno has been influenced by some dark forces that made her turn against Naruto. It could also be that there is some hidden information that led to this sudden change in their relationship.
In fanfiction, Naruto being betrayed by Akeno could be a very interesting plot twist. It might start with Akeno having some hidden motives or being forced by an external power to turn against Naruto. Maybe she was blackmailed into doing so. This could lead to a whole new set of adventures for Naruto as he tries to figure out why she betrayed him and also deal with the consequences of her actions.
Naruto would likely be deeply hurt. He is a character who values friendship and loyalty highly. Being betrayed by someone he might have considered an ally like Rias would be a huge blow to his emotional state.
It's not a mainstream type of fanfiction. While there are fans who are interested in crossovers like this, it has a rather niche audience. The combination of 'Naruto' and 'My Little Pony' with a 'preg' element is very specific and not everyone's cup of tea. So, compared to more common fanfiction types, it's not overly popular.