I'm not sure what kind of reference would work for Beetle offering to trade something for a beetle. I have noted this behavior in game, and it's not one of his normal wares.
Specifically, Beetle will offer to trade one of a select few items that he won't sell normally for a beetle. There's some random element to the trade, but it will always be a consumable which will produce a low level effect of the same variety as the beetle would in a potion. These will range in resale value from 40 rupees to 120 rupees, though most of the time it seems like it's a 50 rupee trade for a Bladed Rhino Beetle, a 60 rupee trade for a Rugged Rhino Beetle, and a 70 rupee trade for an Energetic Rhino Beetle. I've only gotten an item worth more than 10 rupees more or less than the typical for a particular beetle for an energetic rhino beetle, and that was a 120 rupee Energetic Elixir both times.
Mighty Elixirs will, of course have a Bladed Rhino Beetle in their recipe. I was scandalized the first time I saw that... and then realized that beetles really don't live that long, so of course some will occasionally die.
... I should make a table. Tgape (talk) 06:02, January 9, 2020 (UTC)