Why did Moshe refer to the animals that were in the field?
Rashi (on Pasuk 10): The decree was indeed confined to the animals that were in the field, and the G-d-fearing among them took his animals 1 into the house, like the Mechilta says.
Ramban: He mentioned fields because that is where most of the animals were kept. In fact, also those in the houses died as well, like it says (verse 6) "va'Yamas Kol Mikne 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 verse 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 entered also slaves into the house.
Why does it say "Yad Hashem Hoyah"?
Rashi: This is the present tense feminine of 'to be'; in past tense, it is Haysah.
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 in Tanach. It has the letters of Hashem's name, for Hashem was honored through it among all nations.