To answer your question, normally testicles would be noticeable at 5 months old, but there are late bloomers too and sometimes male buns can “suck them up” and you won’t really be able to see them. I’d recommend bringing them to a rabbit savvy vet just to make sure though.

I doubt it’s because they aren’t neutered though. It’s more likely due to a lack of bonding. Even after two bunnies are fixed, there needs to be an “official” pre-bonding and bonding process (which can take from weeks to months). Without this bunnies can fight quite brutally, as you already know.

The first thing you need to do is separate them before one or both hold a grudge against the other due to more fighting, which would make bonding more difficult in the future. I recommend reading the BONDING section of this forum for more info and feel free to ask any questions there

Often when you bring a new bunny home, they are submissive because it’s a new environment and they know they’re in the existing bun’s territory. But after a few weeks, they feel more comfortable and are ready to assert their own dominance – they’re no longer happy to submit to the original bunny just because they were there first. This is why it’s very important to let a new bunny settle in for a couple of weeks before you start prebonding and bonding, so that you see their true personality.

Bonding should be a slow process, where the bunnies build up trust over time. You have skipped lots of important steps, so it’s not surprising that there’s been a fight. You now need to keep them separate for at least a month while you ‘prebond’, where you swap them between cages so they get used to each other’s scents. Because blood has been drawn, one may well hold a grudge, in which case you might need to keep them totally separate for 3 months to forget each other, before starting the prebonding. After a month of prebonding, you can start very short bonding sessions in neutral territory.