Larkspur

Toxic to dogs, cats, and horses per ASPCA: toxic principle is diterpene alkaloids, affecting flowers, leaves, seeds, and stems (essentially the whole plant). Clinical signs include neuromuscular paralysis: constipation, colic, increased salivation, muscle tremors, stiffness, weakness, recumbency, and convulsions; cardiac failure and death from respiratory paralysis are possible in severe cases. ASPCA notes the plant generally becomes less toxic as it matures, and that horses typically don't eat toxic amounts unless suitable forage is lacking: but this is one of the more seriously toxic common flowering plants, and NC State Extension independently confirms toxicity to dogs, cats, and horses with specific named alkaloids (delphinine, ajacine, and others).