All the people that say no way have NO imagination.
Now THAT is some seriously jumped exaggeration.
Prehistoric DLC was already discussed so many times, so I'll focus on the Mythical/Fantasy DLC idea. My answer is: it's not worth it. Now let me explain it.
1) Modern zoological parks don't have space for them.
Mythical/Fantasy creatures are either completely made up, based on poor knowledge, or misunderstanding. Unicorn, for example, would be in last category; misunderstood rhino.
Modern science have defined most of the misunderstanding of the past, and Planet zoo is modern zoo build & management game, so most mythical creatures' groundworks are too fragile to fit.
2) What'll be the purpose of them?
What's the purpose of adopting & breeding fire-breathing, big-as-mountain, claws-deadly, magic-using, flies-whenever-it-pleases, mean-greedy looks-like-reptile in the enclosure? Sure it IS entertaining, but does that match the other purpose of the modern zoo: preservation and education?
3) Zoopedia
Imagine that PZ team decided to make Mythical DLC. What could zoopedia say? What's the scientific name of the unicorn(Let alone that they gotta decide WHICH unicorn they're making, since the 'one horned legendary beast' story is spread around the world)? What is their "wild" behavior? The groundwork of the mythical creature is not only scientifically fragile, but also culturally fractured. Devs have to make up most of the things, which won't happen when they are making real, well studied animals.
4) It's the matter of resource, again.
We don't really know how much longer PZ would be supported. Will making a DLC based on imaginative animals worth more than making a DLC based on real-life-wonders? Resource could be invested in making fantasy DLC can be used to make other DLCs, more fitting to modern zoo simulation game.
'If you don't like it, just don't buy it' statement cannot answer that question.
+) Now, about the mod. I disagree with
The reason so many people still play Skyrim and Fallout years later is because of the mods not because they are such great games.
If they weren't great, neither there'd be that lot of mods, nor they had still-active fan community. Do not miss that. RCT is still played by many, is it because of mods? Not really. Mods do enhance the gameplay, but the game itself is what people enjoy, and keeps the community alive.