(Oh, good lord; I’ve opened a can of worms here – just for future reference, if everyone’s going to start asking me this, I will answer all of them, but if I get a lot it may take me a very long time; after all, “what do you think of such and such a Pokémon” is normally a question I spend three days writing a 1500-word entry on)
Now, Forretress.
I like Forretress. Well, I mean, I don’t actually like Forretress as a matter of personal taste, but I think Game Freak did a good job on him – and particularly on Pineco. They’re Bugs, but they don’t look like bugs, because they hide themselves within constructed shells, like bagworms or casemoth larvae. In fact, Pineco looks like he should be a Grass-type, which plays with our expectations in a way that also references the inspiration for the design. They have an amusing personality too, exploding at the slightest provocation, which makes sense given their obvious desire for isolation. Forretress even has cannons on his body to keep people from bothering him! Basically, they’re bagworms, but not just bagworms; the designers have played with the idea in an interesting way. I probably don’t need to explain in much detail that Forretress is also very useful in battle for laying down Spikes and Toxic Spikes, or Rapid Spinning away your opponent’s. He’s just very well put together all around.
As for Zap Cannon… hmm. This isn’t something he’s always had; in Gold and Silver he didn’t learn Zap Cannon, and couldn’t use the TM either. He only gained it in Fire Red and Leaf Green. That means Game Freak deliberately looked at Forretress’s movepool and thought to themselves “hmm… something’s missing here… I know! ELECTRO-BLAST!” What’s doubly weird is that Forretress isn’t even very good at using special attacks (although Zap Cannon can be useful to him as a sort of poor man’s Thunder Wave) so they must have thought it was really important to his design objectives. Maybe they just thought he should have an attack with “cannon” in the name? I can respect that; moves with “cannon” in the name are awesome. Given the context, though, I have to wonder whether Spike Cannon (which Forretress inexplicably doesn’t learn, despite having cannons that shoot spikes) wouldn’t have been more appropriate…
