D&D 5E - Clever uses for Polymorph spell
Another thing you have to consider is that your blue whale is likely to LITERALLY EXPLODE from the impact. It's a weird trick of physics. You could throw an ant out of an airplane and it would land unharmed. It's just not big enough. Throw an elephant off of a skyscraper, it'll explode. If you take a blue whale, a creature that can only exist at its size because it is in the ocean, any height you could drop from without your gryphon being riddled with arrows before the drop point would lead to a complete structural failure--a meat explosion. That would not be survivable for anything polymorphed into the whale, imo. If you care about these riders and their gryphons, which, given you have to be within 60 feet to properly cast polymorph so the rider is probably the caster, you likely do.
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