Level 17 fighter (561hp)
Level 20 fighter Epic 5 (1038hp)
Owlbears have cleave, great cleave, trip, and intimidate
Yes, you can summon both owlbears and the panther and a vendor hireling (don't feel like posting pics).
Just did it myself inside Invaders!
They are summoned with a different clicky for each (same as Panther) indicating the level (similar to vendor hires). Sadly, they are both named Owlbear Defender (not Jr. or Sr.).
Description on sale page of Owlbear is incorrect if it calls it a level 24 hireling. Owlbear is a 25 Epic if you examine it.
ML for summoning Level 17 Owlbear is 15.
ML for summoning Level 25 Owlbear is 23.