Perhaps they had been a little homesick so far from stables and barns.
"One--two--three--four--why, they are not all here!" Theodora said.
"Here are only seven.

Lib isn't here, or Mrs.Kennard's Sylph." "Oh, I guess they're not far off," Addison said, and began calling, "Co' jack, co' jack!" He wanted them all there before he dropped the salt in little piles on the grassy greensward.
But the absent ones did not come.

Ellen ventured the opinion that they might have jumped the fence and wandered off.
"Oh, they wouldn't separate up here in the woods," Addison said.

"Colts keep together when off in a back pasture like this." But when he went on calling and they still did not come, we began really to fear that they had got out and strayed.
"Let's go round the fence," Addison said at last, "and see if we find a gap, or hoofprints on the outside, where they have jumped over." He and Theodora went one way, Ellen and I the other.

We met halfway round the clearing without having discovered either gaps in the fence or tracks outside.