And, one more small sample of what’s coming in the Matter build in terms of demons, with Geminimax’s sprites. 
(The ones in the post before were using his sprites too, but I uncharacteristically neglected to mention that! Apologies!)
There are three other characters in the Matter build besides these fifteen from the last few posts, but they’re uniques so I don’t want to give them away, at least not yet. 
How many old demons will get new matter abilities? Also fireproof (no fire weakness) plants?
Some Matter abilities are definitely showing up on older demons. For a couple of examples: Slimes will now use a Matter melee touch attack rather than Heavy Blow, and Dryads will have access to an ability that can entangle their enemies.
I think in total about 7 or 8 existing demons ended up with Matter abilities, so in total there will be about 25 or so characters with Matter abilities.
As for the lack of Fire weakness on some plants: it depends on what region they come from. Since both of those plants come from hotter climates, I went with Ice over Fire for a weakness. A bit of a stretch? Of course.
But, resistances sometimes have to stretch a bit for balance reasons: if I was being super-accurate, Fire weakness would probably be way too common.
Also Peluda looks like a sheep with leaves instead of wool. Weak to slash but resistant to other physical damage, does it wear skin made of tinfoil or something?
The slash weakness is in reference to its tail which according to a part of the legend not referenced directly in the description was its only vulnerable point, so it was killed by chopping off the tail. The dense spines protect it from other forms of physical attack.