And another mystery - some of these unrealistic animals - that's in the engravings - seem to be half human.

红色部分是定语从句，应为：which are in the engravings.

去掉破折号，全句的结构是：

And another mystery is that some of these unrealistic animals which are in the engravings seem to be half human.

另一个谜团是，这些雕刻上的不真实的动物似乎是半人半兽。