You cannot select any “Desertapir” with “Desertapir”’s effect.
If you have “Destruction Punch” and “Desertapir” is attacked by a monster with ATK lower than “Desertapir”’s effect, and “Desertapir”’s effect flips the attacking monster face-down, the effect of “Destruction Punch” is applied and the attacking monster is destroyed.
If “Mystical Knight of Jackal” or “Ghost Knight of Jackal” attacks, and is flipped face-down by “Desertapir”, the attacking monster is no longer face-up when “Desertapir” is sent to the Graveyard (which is when their effects activate), so their effects do not apply to “Desertapir”.