How about the saints under the alter?
Symbolic....like the horses mentioned earlier in the chapter....Rev. 6:1-8. Remember, John is having a vision of future events, chapter 1:10.
Matthew 24:9-28 describes the great tribulation "yet to come". This is basically the 5th seal spoken of in verse 9 of Revelation 6.....a future event.
This is akin to Abel's blood crying out Genesis 4:10. It is symbolic also....as blood does not cry.