I've written the English version of the spell as a modified version of an invocation of Poseidon (originally to help in casting a spell) I found on a Wicca web site, adding in the bit about horses since it's an important plot point. I have no idea how authentic the result is:
Poseidon, god of the sea, I pray to thee to aid me in ending this spell.
Almighty you are, yes indeed, with powers of the ocean at your command, you are the master of the sea. I pray the grace of your help to end this storm, and offer this gift of a horse, for you are their master.
So mote it be!
Long live Poseidon, the god of the sea!
I fed that into Google Translate - originally "mote" wasn't translated, so I changed it to "must", after which I got this:
“Ποσειδώνας, θεός της θάλασσας, προσεύχομαι για σένα για να με βοηθήσει στον τερματισμό το ξόρκι. Παντοδύναμος είσαι, ναι πράγματι, με τις δυνάμεις του ωκεανού στην εντολή σας, είστε ο πλοίαρχος της θάλασσας. Προσεύχομαι τη χάρη της τη βοήθειά σας για να τερματιστεί αυτή η καταιγίδα, και να προσφέρουν αυτό το δώρο της ένα άλογο, για εσάς είναι ο αφέντης τους.”
“Έτσι πρέπει να είναι!
Ζήτω ο Ποσειδώνας, ο θεός της θάλασσας!”
Does this look vaguely plausible, or is it nonsense? And is there something genuine that I could use instead, bearing in mind that the goal is to stop a storm via this sacrifice?
Later - the comments have convinced me to go with English text instead, the Greek won't add to the story. And I've found a suitable Homeric verse to quote/mangle here:
http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.01.0138:hymn=22