It's very common for animals to be portrayed by multiple animal actors.
Hedwig, for example, was played by eight different owls. And she mostly just sits in a cage.
Animals are much harder to reason with than people are. If one decides they don't want to work today, there's not many tools that a trainer can use to change their mind. You can't exactly threaten to dock their pay, or sue them for violating their contracts.
Having more animals around makes it possible to substitute a stunt double for an actor that is having an off day.
Additionally, because animals can't communicate their wants and needs as clearly as people can, it is more important to take care of them and anticipate the stresses they are experiencing. A film schedule is stressful for all involved, human or animal. Using multiple animals for each character allows them to take more frequent breaks than their human counterparts, keeping their stress levels manageable.