For example, how do you think they will introduce the first unicorn Poitou or New Forest?

Usually nowadays this is done through promos or pass sales offers

Regarding the first unicorns ever, I've been around the game since 2009 and by then I recall unicorns already being present in Howrse, so I'm not quite sure how the very first unicorns were introduced in the game, but I believe it should have been something similar, probably pass offers

That would make sense. I was there long ago as well but my memory seems to be faultier than yours ahah. Thanks for the answer!

I've been playing on and off since 2007.
They were released promotionally in the old days, as prizes for pass packs, and sometimes given as prizes on social media. I distinctly remember getting them for doing surveys as a newer player as well--usually by the 30 day mark--and I have had this happen within the last year or two on one server, I think NL? So that's not something that's been phased out most likely.

They usually introduce new unicorns either by events, some promotional offer or buy purchasing x amount of passes.

Thanks for your answers everyone!! Hopefully I will get to experience a unicorn promotional offer in the not too distant future hihi

Marie, it's actually your lucky day. They introduced Poitou Donkey unicorns in the Christmas Packs! Literally within the last few days.