| Eastern Copperhead |
The image shows a venomous eastern copperhead (Agkistrodon contortrix). I think it appears flat and wider because it's in a defensive position. Of all poisonous snakes in Texas, these command more of my respect because of how they behave. It can sit in plain view in the leaves, and you'll never know it's there until you step on it or accidentally pick it up if you're doing yard work. I almost picked one up one time. It sits still, possibly, because it's an ambush predator. Wikipedia states that "Copperhead bites account for half of the treated snake bites in the United States."
Image Source: Meta via Texas Snake Identification.