it is required in the diets of humans and other primates, but only of a few other animals.

这是并列句的省略，省略的成分都可以从前一个分句还原。

= it is required in the diets of humans and other primates, but it is only required in the diets of a few other animals.

维生素C为人类和其他灵长类动物饮食中必须的，其他动物只有很少一部分需要维生素C。