Haha, yeah going into Fallout-like wackiness is a question of choice. I, for one, can really have fun without super intelligent animals. And hunting could be really cool if there will be more animal reactions than just attack or run. Like some hidden foes, waiting for prey to come close by... and animals attacking each other. Living, bleeding world, for once
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And about elephant-like entities - I was more thinking about the mutated omnivores, which are easier to feed even with large body mass. Also I really think that some kind of mounts should be introduced - not really for players to ride, but to maintain the story. The apocalyptic world feel. After any disaster which destroys most of technological civilization people will return to using organic means of transport and/or warfare. Technology degrades with use. Without means of replenishing it other ways should and will be invented again.
Just saying
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PS. Considering the misuse of mutagens such large, armored beasts could be used in the Underrail past as organic weapons (a'la Resident Evil world). It may be a path to be considered
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