That headline confused the heck out of me, but that was an interesting read.

It's very weirdly worded. Clearer would be: A type of frog which was once used as pregnancy test escaped and colonoised Wales for 50 years.

Came here to say this! So many ways to parse it. But the key issue was not knowing that “pregnancy-test frogs” was the central concept. Without that, the headline is so confusing… at some point I had to consider whether “frogs” was being used as a verb