Oooh, an excuse for me to express some of my dinosaur knowledge.
First of all I feel the need to clarify that "Therizinosaur" is the term when referring to an entire clade, Therizinosauria. I'll assume you're actually referring to a specific, most notable, genus; Therizinosaurus, rather than an entire clade where the specific members can look pretty different from each-other.
Second of all, our most accurate reconstructions of what Therizinosaurus probably looked like is a five ton, long necked, giant clawed theropod.
Does this look anything like Combusken to you?
Also, I feel the need to express that people seem to think we know species exist because we discovered whole skeletons of extinct specimens. In reality, fossilization is actually a very rare phenomenon, conditions have to be just right for it to occur and even if it does then the fossils will probably degrade before anyone finds them. The dinosaur skeletons you see in museums? Yeah, not actually bones -- they're replicas made from the casts of bones because the real bones are too fragile to display. More to the point, paleontology isn't always as fulfilling a field as people sometimes assume. Fact is, we only know certain species exist thanks to a couple of fragments of a fossil, and the rest is completely up to the imagination. That said, these are currently the only known fossils for the Therizinosaurus. Notice there is no beak. Saying Combusken looks like Therizinosaurus just because it has claws and a long neck is like saying it looks like a flamingo for the same reasons :p
I do give you props for the fact that Therizinosaurus is a member of the Maniraptora clade, which are the few genus' of dinosaurs that did actually evolve into birds.
If there was a Pokemon based off of Therizinosaurus, it'd probably be a fossil Pokemon. That'd be awesome because I love Therizuinosaurus -- but Combusken ain't it it.
And now I'll wait for this to be closed and my super long answer to be forgotten in time.