Hopefully never. What's even the point of adding it, if you change it to some fictional monster in the first place?Regarding Segisasaurus, if it were to be added to the game, I think it might be for the best if it were to have a visually unique feature (similar to Dilophosaurus' frill and Deinonychus' fin) such as a cobra's head like in this illustration:
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Then you might just as well add some other species that are unique by themselves naturally, and not that kind of "unique" by adding some fake, made-up features like that.