amok: Atari might not have the rights to it anymore, they where the publisher of the game, not the developers. Depending on contracts, the rights may have reverted back to the developer. If this is the case, it was created by the then Woedend! Games. This company underwent several name changes since then, but ended up as Force Field Entertainment, which was one of the dvelopers bought by Embracer Group when they did that big spurge back in August. So if this is the case, then the rights are with Embracer Group now.
If Atari do kept the rights (as My Horse & Me was released in 2007, after Atari was broken up and the corpse was bought by Warner), they (i.e. Warner) should still have the rights, but they just don't care about it, or do not think it is worht the investment making a digital version. This is can be understandable as it did not sell very well, and did not get great reviews.
It was also supersided by My Horse & Me 2 / My Horse & Me: Riding for Gold (yes, this is a games series....) in 2008/2009, which is basically more of the same but a bit improved.
Bottom line, I doubt Atari or Embracer Group will re-relese this game. It is too niche, there is not a large enough market for it. You need the subsection of users who a) want a game like this and b) don't mind older games. If anything will happen with this series, then most likely it will be that whoever has the rights will make a new game with the name, thouhg i do not see that happening either.
I was trying to find out which company has the rights to this game. W! Games changed its name to Vanguard Games, later to Force Field Entertainment, and now it is Vertigo Games - Amsterdam. And no, they no longer have the rights to the game.
After 2009, Bandai Namco Partners gained some rights to Atari games. But in 2013, Baindai Namco Partners changed its name to Bandai Namco Entertainmnet Europe. It was the distributor of games such as Test Drive Unlimited 2 or The Wichter 2 in Europe. I have contacted them and they also no longer have rights. It seems that only Atari must have the rights. If more than one player asks to re-release the game, it may be possible to restore it.
I rather doubt THQ Nordic has the rights to My Horse & Me.