YOU SAID:

I was riding my horse in the field with my dog

INTO JAPANESE

私は犬と一緒に野原で馬に乗っていた

BACK INTO ENGLISH

I was riding a horse in the field with my dog.

INTO JAPANESE

私は犬と一緒に野原で馬に乗っていました。

BACK INTO ENGLISH

I was riding a horse in the field with my dog.