In Ezekiel 26:11, this signifies the love of learning or intellectual things. (Arcana Coelestia 3727)

In Revelation 6:2, this signifies the love of understanding the Word. (Apocalypse Revealed 298)

In Zechariah 12:4, this signifies that the intellectual should be filled with falsities. (Arcana Coelestia 2383[2])

In general, 'a horse' signifies knowledge or understanding of the Word. In an opposite sense it signifies the understanding of the Word falsified by reasonings, and likewise destroyed from self-derived intelligence. 'A dead horse' signifies no understanding of truth from the Word.

508. Verses 8, 9. And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea; and the third part of the sea became blood. And there died the third part of the creatures in the sea having souls; and the third part of the ships was destroyed.

8. "And the second angel sounded" signifies influx out of heaven, and the second consequent change with the evil (n. 509); "and as it were a great mountain burning with fire," signifies the love of self and of self-intelligence therefrom n. 510; "was cast into the sea," signifies into the natural man (n. 511); "and the third part of the sea became blood," signifies that in consequence everything therein became falsity of evil n. 512.

9. "And there died the third part of the creatures in the sea having souls," signifies that in consequence every living knowledge [scientificum] in the natural man perished n. 513; "and the third part of the ships was destroyed," signifies that all knowledge of truth and good from the Word and from doctrines from the Word also perished (n. 514).