YOU SAID:

They were riding on a horse when they went up into flames.

INTO JAPANESE

彼らが馬に乗っていた時、炎に包まれた。

BACK INTO ENGLISH

When they were riding on horses, they were enveloped in fire.

INTO JAPANESE

彼らが馬に乗っていると、彼らは火に包まれた。

BACK INTO ENGLISH

As they rode on their horses, they were enveloped in fire.

INTO JAPANESE

彼らが馬に乗ると、彼らは火に包まれた。

BACK INTO ENGLISH

When they rode their horses, they were caught in the fire.

INTO JAPANESE

彼らが馬に乗ったとき、彼らは火事に巻き込まれた。

BACK INTO ENGLISH

When they rode their horses, they were caught in the fire.