YOU SAID:

they must have undergone the equine therapy

INTO JAPANESE

彼らはウマの療法を受けたに違いない

BACK INTO ENGLISH

They must have had equine therapy.

INTO JAPANESE

彼らはウマの治療を受けたに違いない。

BACK INTO ENGLISH

They must have been treated for horses.

INTO JAPANESE

彼らは馬の治療を受けたに違いない。

BACK INTO ENGLISH

They must have been treated for horses.