It’s a Chimera
The question was put to Lucasfilm Story Group leader Pablo Hidalgo in a recent episode of Rebels Recon:
ANDI GUTIERREZ: I’ve got a question from Joshua Parish. He asks "Do the insignias Thrawn’s stormtroopers have on their pauldrons and uniforms have any significance?
PABLO HIDALGO: They do. They are a stylized depiction of a chimera.
Basically a three-headed monster. So that’s what those prongs
represent. Basically anyone in Thrawn’s forces has the ability to wear that emblem.
As mentioned in another answer on this question, this is indeed the name of Thrawn’s ship, as mentioned in another episode of Rebels Recon:
GUTIERREZ: So, Jake Anderson asks, “What was the artwork I spotted
under Thrawn’s Star Destroyer in Warhead? Does it have any meaning?”
HIDALGO: Uh…let me think about—yes! It is the Chimera; that’s
the name of Thrawn’s ship, and so on the belly of the ship he’s got a
stylized version of that creature.