YOU SAID:

There was a gas moving horse

INTO JAPANESE

ガスが動く馬がいた

BACK INTO ENGLISH

There was a horse with gas moving.

INTO JAPANESE

ガスを積んだ馬が動いていました。

BACK INTO ENGLISH

A horse loaded with gas was moving.

INTO JAPANESE

ガスを積んだ馬が動いていた。

BACK INTO ENGLISH

A horse loaded with gas was moving.