Here's a prey that's simple but challenging: Smug Slime
A slime girl with a short shirt saying "drink me" and her face is smug as if she's saying "c'mon. I dare you. Bitch. Just see what happens"
She's worth a ton, but upon digestion the pred loses digestion (or capacity if you can work around glitches like over capacity or no capacity
The idea is to present a challenge to well established fields (that the right word?)
This leads me to another idea for consideration.
Here's some ideas I have on the prey deck. I mention these because it touches on the previous idea a little but also might expand on future idea possibilities
Some prey only show up at certain parts of the game, or at least are more likely to.
This is to keep troublesome prey (like Smug Slime) from screwing players over early on.
Some prey could show up only early on to make catching them more challenging.
A special store upgrade might make certain prey show up early on to help build your field quicker.
Then there could be super rare cards that only have a chance to show up on every tenth turn.
*edit* last idea here: all I'm gonna say for now is BOSS CARDS. Not gonna get into how they work now. Just gonna leave it as food for thought
Hope I didn't ramble too much. If my ideas are unclear, don't work with what you're trying to do, or just make a mess out of the code, feel free to let me know. I want to be helpful