Why did Moshe refer to the animals that were in the field?
Ramban: He mentioned fields because that is where most of the animals were kept. In fact, also those in the houses died as well, as it says (in Pasuk 6) "va'Yamas Kol Mikneh Mitzrayim."
As the Torah writes later (in Pasuk 20) in connection with the plague of hail. Refer to 9:20:1:1. The Mechilta applies it also to Dever. (That Pasuk mentions also slaves. Perhaps it refers to Dever only regarding animals. Or, masters feared that also people in the field at a time of Dever are prone to die, so they brought the slaves as well into the house.
We saw earlier (in 8:15) that each plague is referred to as "Etzba Elokim" and the miracle of te Yam-Suf as "Yad". Why does the Torah refer to Dever as "Yad Hashem?
Oznayim la'Torah #1: Refer to 8:15:2:3*. The Torah refers to each Makah that struck the Egyptians themselves as 'Etzba Elokim', whereas Dever was confined to the animals
See Oznayim la'Torah.
What are the connotations of the term "Dever Kaveid Me'od"?
Oznayim la'Torah (in the name of the Midrash Seichel Tov): It was described in this way because not a single animal survived the plague.
What are the connotations of "Yad Hashem Hoyah"?
Rashi: "Hoyah" is the present tense feminine of 'to be'. The past tense would be 'Haysah', and the future 'Tih'yeh'. 1
Moshav Zekenim: This refers to something sudden. The animals were healthy, and suddenly they died.
R Tuviya, in Chumash ha'Rishonim: There is nothing like this word (Hoyah) in Tanach. It has the letters of Hashem's name, for Hashem was honored through it among all nations.
Rashi: like Osah, Rotzah and Ro'ah.