Ok before I hit the hay. You can turn into an animal at will. Your clothes and such won’t turn with you but mass is not conserved so you can turn into something much bigger or smaller. It has to be a real creature that does or has existed and I guess if you want a plant that would be fair game. For me its an albatross. could travel around the world. might even manage to carry something that is not to large if packaged right. I am a bit torn though as the ocean is large so maybe a sea creature would be better. like a killer whale. of course then its not great for getting somewhere inland.
Lobster.
I’d continue to live my normal life during the day, but transform every night to sleep, and don’t age during that time. Hell, any time I’m not doing something that requires a human body, just push pause on the aging process for a few hours. Careful application could get me an extra ~40-50% lifespan for free.
The lobster’s biggest problem is always having to grow new shells. If every transformation puts me back exactly where I left off, that could be an issue, but with a part-time human to stock my own tank/pool, getting enough to eat as a lobster to get the energy to do that shouldn’t be an insurmountable hurdle.
Well you didn’t add any caveats to the transformation and you said it’s at will so… If mass is not conserved then I’d just turn into something heavy yet compact like a rhino and harvest infinite energy from the toggling. If I get support from a State I might even choose a whale so we can extract the most energy per toggle. You can basically build an engine from it, and since whales are like 200 tons and I’m 70kg, the conversion ratio would be pretty damn good. Though it would require a dedicated facility, which is why I’d rather go with a rhino or something, which is however about 100 times less efficient so it would require 100 times as much toggling. You get the picture.
That sounds like slavery with extra magic.
Others: I like this animal for reasons.
You: I’ll be a power plant.
