There's obviously quite a lot of CMF parts and accessories that have made other sets better, do you mean CMFs released almost concurrently or with in a few months of release that the designers intend to complement the set? As in your foal example?
I'm going to quote myself from another post :P
When I say "new parts budget" I mean unique elements.
Others I can think of:
Series 19 Monkey King was obviously a precursor to Monkie Kid the following year.
Series 20 Pyjama Girl's bunny was possibly made to include in 21324 Sesame Street or maybe the designer just took advantage of the recently produced element.
Series 21 Beekeeper's headgear, honey comb tile and Ladybug's printed ladybug tile were probably a case of the 41703 Friendship Tree House designer taking advantage of the recently produced existing element. There's a heap of these examples like the Hotdog, Pea and Corn Cob costume being repurposed but still pretty cool.
Series 8 I think the Vampire Bat may have been produced so the elements could be reused for Monster Fighters Vampyre Castle even though the Castle was released first by a few months.
Conversely I think it's interesting when it goes the opposite way. The Centaur body being produced for the Harry Potter 75967 Forbidden Forest, then they were probably thinking what can we do with this one off mould, ok stick a Centaur Warrior in the next CMF series ;)
edit: Oh and the Series 22 Racoon Costume minifig head is also in 10305, but it's pretty generic, they could have reused any of the other female minifig heads.