In multiplayer the cactyrant doesn't use it's nether variant texture but it does use it's nether variant loot table and in singleplayer it does use it's nether variant texture.
EDIT: Cactoids share the same problem too.

I also tested without other mods and I found out if you make a new world on a server and spawn a cactyrant in the nether it won't use the nether variant texture at first but when I restarted the server the cactyrant's texture changed to the nether variant texture.

Also when I test it with my other mods that method doesn't work so I'm going to try to find the culprit, I'll edit this when I do find the culprit.
EDIT: I found the culprit, It's the "Just Enough Resources" mod.

Version 1.4.0

In multiplayer:

In singleplayer: