A photo of the fruit might be helpful... but even then, chances are that all we could tell you is what variety its not.

If you intend to grow it next year, just let a tomatillo or two ripen, and save the seeds. Let them get soft, then break & squeeze them over a tall container of water, and beat vigorously with a wire whisk. You can also place the unhusked fruit(s) in a blender 1/2 full of water and pulse the contents at the lowest speed a few times, just until the fruits are broken up. The seeds are very hard, and most will be unharmed. Either way, the good seeds will sink... the bad seeds & debris will float, and can be skimmed or poured off. Then drain the good seeds through a fine wire strainer, and spread them out in a single layer to dry. I spread them on newsprint & stir them around (to wick off excess water) then move them to a smooth surface to dry.

Don't know where you are located, but tomatillos grow from seed even here, even if not started indoors. I have volunteers growing now from where my DW threw some rotten ones last year. Chances are good that you will have plenty of volunteers in that location even if you do nothing, from fruits which fall to the ground & rot. You may even find volunteers in other places, where mice have spread the seeds. ;-)

Wow — that's a beautiful tomatillo plant!!