9 Succulent Plants Toxic to Cats, Dogs, or Pets Although aloe vera is known for its many medicinal and beneficial properties to humans, it is toxic to both cats and dogs when ingested. Euphorbia Tiruacalli (Firestick, Pencil Tree Plant, Pencil Cactus)Toxicity: to dogs, cats, horsesEuphorbia is a large genus of succulents with over 2,000 species.