Gasterophilus, "bots" hatch within a week or two. The larvae is carried to the lip or jaw hairs and can also be tranferred to the mouth by licking of the legs hairs. They stay in the mouth for about a month and develop to the second stage before heading to the stomach. This trip usually takes about a month from mouth to stomach. In the stomach, they develop into their third stage and attach to the stomach or upper part of the small intestine and stay there for 8-10 months befire being passed in stool. Therefore, I would rule out the bot that you have just found from coming from your yearling filly. This does not mean that they are not in development in her at this time, it is just unlikely that they have gone through their life cycle within her life span, to this point.
Trichlorfon (Combat Liquid, Combat Paste, Negabot Paste, Telmin B, Benxelmin Plus, or Dyrex), Dichlorvos (Cutter Diochlorvos), or Ivermectin are all effective for bots. Some require special information for their use, so consult your veterinarian for the best method in the case of your herd. Especially with consideration to your bred mare. Hope this helps.
Edited to add: Dictyocaulus, "lungworms" are not usually found in horses, however it is more common to see them in horses pastured with donkeys, as Genie pointed out. The good news is that there is little resistance to the benzimidazole wormers (much less resistance than other types of worms have to the wormers in the benzimidazole group, which chemical names usually end in 'dazole'), so Ivermectin should kill those off without too much problem.