Let's say as a party you've taken down a few monsters already. Without knowing the proportional strength of a chimera to the monsters you've already faced how would you know?

Like let's say I did kill an Orca on my own. Then I see a polar bear, the Orca was a lot bigger so seems reasonable.

Now living in the age of technology, not a fantasy setting, I personally know that a polar bear would be a lot harder to beat. But how's an adventurer supposed to know that? They don't have wikipedia and national geographic in most fantasy settings. It's not unreasonable to believe when encountering a creature that no one in the party is familiar with it at all.