A beautiful Fauna cover from Belgium, depicting 2 stamps on topic "Endangered mammals in the North Sea". Both stamps are part of a set of 5 stamps issued in 2017 and the both have a special cancellation of first day issue.
A nice Fauna cover from Belarus, depicting a complete set issued in 2017 on topic " Mammals of Belarus". There are four species of Mammals represented on this set: the Hamster (Cricetus cricetus), the stoat (Mustela erminea), the Badger (Meles meles) and the Brown bear (Ursus arctos).
A very nice WWF cover from Finland, depicting a complete set issued in 2016 with three stamps with endangered species: the lesser white-fronted goose, a species of damselfly and the saimaa ringed seal. What makes this cover so special is that it was posted and cancelled at Santa Claus Post Office in Arctic Cercle (finish postal code 96930). https://www.santaclausvillage.info/santa-claus/santa-claus-main-post-office/