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Spontaneous 4/4s have been okay-to-awkward blockers ever since Wings of Velis Vel, and this Dragon-specific version is no different.
Source: FilipaMTG
(Aside: I'm not sure if Mordenkainen has a known spell like this. Maybe it's something he never taught, or a variant of a spell pulled by his involuntary wizardry spell. What I am sure of is the art is showing Mordenkainen's faithful hound.)
that's where I know "Mordenkainen's Magical Watchdog" from.