Ok thanks for the info.
If it was just the one horse, I would definitely think it could still be related to the tick bite. Especially an older horse, has some trouble dealing with appropriate lymphatic drainage after an inflammatory episode and if there was any part of the tick left that could explain a recurrence.
With the 2 horses...It still could be related to ticks...check them both very closely again. There is both the local irritation from the bite and the risk of systemic illness. I would expect with tick borne illness such as anaplasmosis you would also see fever, diarrhea, or a depressed horse. Assuming you have checked them both very closely to make sure there are not any other ticks attached to the scrotum or penis or anywhere else then we get into other possibilties:
1. Allergies or weeds...This time of year isnt huge for allergic reactions but we do still certainly see them if there are weeds in the hay
(sometimes it just takes one bite). Or if you do still have ground that isnt frozen, then they could have ingested a weed in the pasture (if they are out). Hoary alyssum is a common one here in Minnesota that can cause stocking up, fever and founder.
2. Lack of exercise/movement combined with colder weather: In older geldings, we very often see edema (the swelling) in the sheath when the temp drops or there is bad weather and they arent out moving around much. If they are a little overweight then it makes it worse. Again this has to do with poor lymphatic drainage. The solution is to get them moving (10 min of trotting work once per day for 3-4 days should help)
As long as they are eating, drinking, peeing, pooping (no diarrhea) and no fever (temp less than 101.5 F) then I dont think you need to worry too much about them. If you have some bute or banamine I would give each a dose of one or the other of those. If the swelling doesnt resolve with some exercise, time, and antiinflammatories (the bute or banamine) or if they get a fever, develop diarrhea, or go off feed then they should be seen by a vet.
I hope ths has been helpful. Feel free to ask a follow-up if have any other questions.