Tried multiple copies of forge from 14.23.5.2870 to 14.23.5.2807 [and older]
Including the stable 14.23.5.2768 release
and BabyMobs-1.12-1.5.5.jar only.

Not much of a problem with just forge and BabyMobs running but on a server running more mods it becomes quite a pause which corresponds to the message on the client side log...
[1.12.2] Received passengers for unknown entity
When I check the server log no issues are recorded at the same time frame.

Hope this helps

OK, so "[1.12.2] Received passengers for unknown entity" is not necessarily the problem. Turning on mods one at a time to see what other mod [or combination] makes "[1.12.2] Received passengers for unknown entity" turn into a pause instead of just another warning. Maybe figure out the why.

Had to get rid of some other mods that add mobs. Seems there mods did not handle the problem gracefully causing massive lags. Still have BabyMob, Mowzie and The Gaia Mobs with no problem, still get the message but not the massive lag. I never nailed down which mod specifically was the problem as I turned mods on if the problem returned the mod was dropped so it seems to be a body of code [a standard used] that causes the problem and something inherently wrong with riders. Calling this closed and not a BabyMobs issue.