Caesar uses 'ad equum' to mean 'turning them into horses'. This is a famous quote used in OLD itself. Is this an idiom? There is no reference to 'converso' or 'mutatum'... just 'ad' although habiturum is lurking close by... Like 'having them turned into horses'.
I assume you mean this quote:
Quod cum fieret, non inridicule quidam ex militibus X. legionis dixit: plus quam pollicitus esset Caesarem facere; pollicitum se in cohortis praetoriae loco X. legionem habiturum ad equum rescribere.
And when this was done, one of the soldiers of the tenth legion said, not without a touch of humor, "that Caesar did more for them than he had promised; he had promised to have the tenth legion in place of his praetorian cohort; but he now converted them into horse." (trans. McDevitte/Bohn)
It does not mean that he turned them into horses! That would be an act that even the divine Caesar couldn't do.
[Edit: Thanks to Sebastian Koppehel for the clarification.]
The "horse" in question is actually cavalry status. Caesar went above and beyond to reward his troops, and instead of making them his bodyguard (cohortis praetoriae), he made put them on horses.
I'll quote Sebastian below:
He literally put them on horses, because he trusted them most and Ariovist demanded that Caesar bring no infantry to their meeting. The unknown soldier remarked that Caesar was really generous by using them not just as his bodyguard, as promised, but making them all knights.
Grammatically, the verb in question is rescribere, which has a special military usage meaning "transfer". In fact, Lewis and Short quote this very passage for that meaning:
- [select] In milit. lang., to transfer from one kind of troops to another: “Caesarem decimam legionem ad equum rescribere,” Caes. B. G. 1, 42 fin.
So they're not "turned into a horse" with any ordinary word for change, but are transferred from legionaries to cavalry.