Mr. Caxton Draws a Martian Bird by Frank Belknap Long

On the harsh Martian frontier, young Peter and Susan are left in the care of their impatient guardian, Mr. Caxton, while their parents explore the desert. The curious children spot mysterious clawmarks in the red sand, but their discovery is met with scorn. As night falls, an otherworldly creature materializes at their door, igniting terror and wonder.