If you have more than one mutagen from Zulrah you should be able to put both mutagens on the same serpentine helm, giving it the same transform options as the snakeling pet (
Serpentine helm | Old School RuneScape Wiki | Fandom
Recoloured Serpentine Helms : r/2007scape
Serpentine Helm or Anti-Venom+ | which one is better for Zulrah ? - YouTube
Making Slayer Helm Osrs - Wiki - Free Transparent PNG Download - PNGkey
This Headless Horseman's Helm paper model in 1/1 scale was created by designer Harlan Browning. Inspired by a 3D mode… | Paper models, Headless horseman, Horseman