It is not that your animal is aggressive, it is just showing aggressive behaviours. Aggression is more common than you first might think and develops with good reason. All behaviour is learnt behaviour and that means it can be unlearned. The first step to introducing a new animal your current pet is to make sure you are doing it the right way that creates a calm and positive response. All animals like their persona space and some simply need to have bigger personal spaces than others.
If a similar situation (good or bad) repeats and your pets' reactions to that situation work for them, then that becomes the behaviour response they will continue to rely on. Learn what body language your animal is trying to tell you before the aggression erupts. Some basic behaviours are:
For solutions to solving aggressive behaviour, it is helpful to notice the above general body language cues. Look for patterns and see if you can identify any triggers for the aggressive behaviour. At this stage, we recommend seeking a professional animal trainer so that your animal does not potentially hurt anyone or another animal. the trainer will teach you how to recognise the body language your pet uses to communicate so that you can begin the process of unlearning the aggressive behaviour and replacing them with new, desirable behaviours.
If you'd like to do your own research into more information about dog behaviour and body language, may we suggest looking into these websites: