This Western is the first feature directed by the great director Arthur Penn (Bonnie and Clyde, Little Big Man, The Miracle Worker, etc.) It's based on a play by Gore Vidal, who shows Billy as a misunderstood kid rather than a psychopathic outlaw, as he is in most fiction. Imagine James Dean in the role, because he was supposed to do it, but he died before he could.
Billy sets out to avenge the death of his friend and employer and causes all sorts of trouble. In the lizard scene, Billy is in bed recuperating from some burns when his two friends come in laughing about how the whole town thinks Billy died in a fire. They pull out a box telling Billy that it's an offering to put on his grave - a little friend to keep him company - a horny toad. One guy picks it up and pulls it out of the box to show Billy and they all laugh and Billy pretends to be dead, then they put it back in the box.
They're supposed to be somewhere in New Mexico, where the local large horned lizard is the Texas horned lizard, but they actually used a live Blainville's (Coast) Horned Lizard, which makes sense since the filming was done in California, where that horned lizard comes from.