How can I get the creature's that doesn't spawn?

There are a lot of creature’s that doesn’t spawn,
Baryonyx G2
Allosaurus G2

How can I get ‘find’ this creature’s?

Depending on which, sometimes it is just random incubators while others it’s when they become a dinosaur of the day or daily mission rewards.

Allosaurus G2 isn’t released in the game as yet. Hopefully it won’t be long until it is.

Something I get 15, 20 DNA from the incubators,… i barely unlock them, with this few DNA how can I ever fuse them to legendary or uniques like Manga?

By playing the long game for some of them. For instance, it took me nearly a year of constant playing to complete both Indominus and Indoraptor. Not everything is immediately available.

The only ways to get them are to speed up incubators (which i don’t recommend doing because of the next choice), or to interact with them in sanctuaries. Also, you can request the DNA from your alliance for the rare dinos. It still takes forever with any of those options but is faster than just waiting. I’ve been playing for about 14 months and still only have Magna at 450/750 on level 29. Finally almost done. Unless the fuses start being nothing but 10s.