Covering a stink with a not so bad smell.
Sometimes it works, Good Luck.
I'd have thought, some very soft soap, Lux Flakes, or velvet bar soap, Powdered down with a grater.
Or Eucy wool wash, and just give it a wash in the affected area. There should be a stain.
Otherwise the "JUICES" from 'poor Fredo' will not do the canvas any good.
Make sure you thoroughly wet the area first, and rinse out the soap really well.
You shouldn't need a lot of soap in warm water, depend on how large the frog was.
Hope it wasn't a CANE TOAD.
Dry it out and reproof the Canvas after.
Consult a specialist to find out about the reproofing products.
Scotch-guard 'might' be OK. I'm only saying, as a short term though.
It does come out faster than a lot of other things.
Those other things can be diluted PVA Glue as well.