It is a Grass and Steel-type Pokemon. This is a very unique typing that has only two weaknesses (though one is a 4x weakness) and a plethora of resistances.
How to catch Ferrothorn in Pokemon GO
Currently, Ferrothorn does not show up as a wild Pokemon anywhere in Pokemon GO. Therefore, the only way to get one is to evolve it from a Ferroseed for 50 candies.
Ferroseed is a relatively uncommon Pokemon in Pokemon GO. However, thanks to its current spawn boost from Sustainability Week, it should be easier to snatch a handful of them.
The best bet to secure one is to scour places where Grass or Steel-types frequently pop up. Grasslands and very modernized areas (i.e. cities, factories, railroads) are the best places to find this Pokemon. However, it's also possible that Ferroseed's spawn boost will overpower the biomes system, and it may pop up just about anywhere.
Lures and incense will be helpful in luring out the barbed seed Pokemon. The more possible encounters, the better.
Once the player has managed to track down a Ferroseed, all of the usual tricks will work. They can throw it a Razz Berry and chuck a curved Ultra Ball at the circle to get the best odds at a successful catch.
There will also be a guaranteed Ferroseed encounter during the second set of Field Research tasks for Sustainability Week. Catching 5 different Grass-type Pokemon will net players a single guaranteed chance at a Ferroseed.
If players miss out on Ferroseed this week, their best bet (besides hitting up grasslands/factories and praying) will be for the Pokemon to pop up in low-tier raids or egg groups, as it has in the past.
In addition, Ferroseed has a shiny variant in Pokemon GO, so interested players should keep their eyes open for this chance occurrence.