The Beasts are purchased as wargear but behave like fighters. However, there are a few exception, one of them being that they don't count towards the number of fighter you can select for a given battle (if you take a fighter with beasts, you can also take the beast). It's not spelled out they should not be shuffled with the other fighters when selecting a crew at random or selecting sentries.
RAW, I think they do count as fighters as far as bottle tests are concerned though (both towards the number of total fighters and the number of OoA/seriously injured fighters).