If you have room for them and if you were careful to educate yourself on deerling care before this point, it’s actually possible to adopt them if you wish—and, in certain cases, it may be preferable, as the deerling may have gotten used to you or their current habitat.
Here is what you should do first. Go ahead and show yourself to the deerling. You can offer them food at this point but keep your distance. If they come to you, then it’s a good possibility that they’ve gotten used to your scent and that they would actually prefer to stay with you.
In addition, it’s worth it to consider that the deerling may have become accustomed to your yard in general. To determine whether or not this is the case, take note of how they react to you when you introduce yourself to them. If they seem antsy or run away at the sight of you, call your local Nurse Joy and have her come to look at the deerling, if not send a local trainer to fetch them. These are professionals who will be able to determine whether or not the deerling have become accustomed to your yard. If the deerling have, Nurse Joy or the appointed trainer will give you the option to adopt them. If the deerling aren’t accustomed to your yard specifically, then they will attempt to round the deerling up and take them to a shelter.
Ultimately, in this case, it may be worth it to give the deerling a chance to make a decision. If they consider your yard specifically to be their home, it would certainly be worth it to consider adopting them, yes, especially if your houndoom would be perfectly happy with deerling companions.