Spotted Heart Frog

I usually just breed frogs until I have them all. After I've found a few, I lay them out on a floor somewhere (with no access to them, because sims are obsessed with looking at the things) and see which ones I'm missing.

Any heart frog and any spotted could produce a Spotted Heart. It CAN take a lot of time, though, especially with the cooldown on breeding. And you'll end up with quite a few duplicates too. I'd just sell thoes, or use as bait or plasma packs.