The chasmfiend is the mature, adult form. Two quotes as support:
WoR ch. 49
"They come here to pupate. Have you ever seen a juvenile? Before they form the chrysalis?”
“No,” Adolin said, scooting his chair around the table. “It often happens at night, and we don’t spot them until morning. They’re hard to see out there, colored like rock."
WoR ch. 70
But they come to the Shattered Plains to pupate, and here there’s very little food, which is why they attack men. Why has this one remained after pupating?
I assume they haven't mentioned the "juvenile" forms much because they are hard to spot and catch. By the time they post one and report back to the warcamps it would be gone, unlike the chrysalises, which aren't going anywhere. Presumably if you saw one while out on an assault or a hunt you would try and catch it. But they're probably too fast for humans to catch. The only reason you'd catch an adult is by baiting it, and relying on the fact that it's aggressive enough to come to you. The juveniles probably run away. And if they only come out at night, that would make them difficult to spot in the first place.
That said, I don't think the Alethi make too big of a distinction between juvenile and adult chasmfiends other than the fact that the adults are bigger.
The chasmfiend has been spotted by patrols here a good dozen times. He's got his nest nearby, for certain--he's not here to pupate. He's too big for that, and he's remained in the area too long.
-WoK, ch. 12
In the middle was the contested area, the section of plateaus where the chasmfiends most often came to make their chrysalises. Few came to the near side, where the permanent bridges were. If they did come, it was to hunt, not to pupate.
-WoK, ch. 24
Occasionally, chasmfiends stalked these pathways, searching for either carrion or a suitable plateau to pupate upon. Encountering one of them was uncommon, but possible.
-WoK, ch. 27
Huh, that's a good point. I seem to recall that chulls have two pupae stages. One to mature into an adult and another where they just sort of turn into a rock and die(?). I wonder if some of these might be referring to the adults going through that second pupation?
Ah thank you, I figured it was mentioned somewhere and I just couldn't remember it.
Well that's a bit less exciting for me, I was hoping we kept seeing the juveniles and one day we would spot a 100ft tall chasm fiend. I still want to get the dimensions on the Reshi Isles Great Shells, something that takes an hour to scale from tale to head and has an eye the size of a house must truly be beyond comprehension in size, and the gemheart must be the size of the Palanaeum!