stable companion 
noun
Another term for stablemate.Mrs Ford partnered Forest Gunner in a two-mile gallop at Aintree on Tuesday with a stable companion at the track....
- Celestial Gold's 12-1 triumph came after his stable companion and original ante-post favourite, Our Vic, had been withdrawn earlier in the week with a muscle problem.
- Marlborough is a useful second string for Henderson but Bacchanal looks a cut above his stable companion.