Mark has been strategically placed in panel 1 to keep this strip family-friendly. It is just an oversight, I’m sure, that led Rivera to focus more on Artemis, herself, and less on the proportional relationship of the horse to Clayton in panel 1. No matter, as Rivera straightened out the visual ambiguities in panel 3.

[Edit] Mark expresses his ignorance of horses in general or maybe just the mustangs. I suppose that with Rivera’s reboot, Mark’s pre-Rivera past experience with horses (as noted by a comment from Downpuppy a few days ago) has been wiped clean for the sake of this story. Otherwise, Rivera seems to be maintaining a more serious approach than has been usual in past adventures. That’s good, but how long will this last?