Eric runs in after it,followed a big dog,walking very slowly

Eric runs in after it是主句.followed a …作伴随状语,逻辑主语也是Eric,原型是he is followed by a big dog.
后面的walking very slowly也是伴随状语,逻辑主语是 a big dog,原型是It is walking very slowly.
也可以写成Eric runs in after it.He is followed a big dog.And it is walking very slowly.

第一句是主句

Eric runs in after it,followed by a big dog,walking very slowly.
这是个简单句，主干句为Eric runs in after it
followed by a big dog,walking very slowly.作伴随状语。