If it's about poaching or hunting, it could be the royal "yowe" , a ewe.

perhaps derived from an old pronunciation of the Gaelic for a yew tree, but I don't think that makes much sense in the context of the line.
"agh" makes a lot of sense, as in "agh-fèidh" a hind, a female deer Agh = heifer; in deer "agh" it can be used for a hind or a fawn. Look up "agh" at https://learngaelic.scot/dictionary/index.jsp and click the icon to hear the sound file. You can also look up agh-fèidh or fawn.

",oh" doesn't make much sense either as it leaves the word "royal" hanging. I think "agh" in reference to a deer is most likely.