Mogo was Batman's "Bat-Ape".
Mogo is a trained gorilla from a circus. With his trainer Arthur Harris and his trainer's assistant Ralph Rodor, Mogo puts on shows for circuses. At the annual Gotham Charity Circus, Batman and Robin first meet Mogo. During Mogo's act, he begins an uproar for some reason. Harris is able to calm him down, but not before masked thieves are able to make off with the receipts for the show. Harris is held on suspicion of facilitating the robbery by using Mogo.
Batman and Robin investigate the area, they find that someone has rigged a wire onto the platform Mogo was on that caused an electrical shock, causing Mogo to go berserk. Afterwards, they confront Rodor to tell him about it. Batman and Robin tell Rodor to put Mogo in a larger cage, since the current one was too small. Following this, Mogo chooses to break out of his cage to follow his friends Batman and Robin back to the Batcave.
Back at Wayne Manor, Mogo puts on one of Batman's spare capes to imitate his new friend, much to Alfred's chagrin. Batman, Robin and their newfound partner Mogo trail Rodor back to a warehouse, where he's meeting with the Vanner brothers to split the loot from the circus heist. Although they flee, Mogo is able to trail them by following their scent. With Mogo's help, Batman is able to push a large globe towards the crooks, cornering them. With the crooks cornered and caught, Harris' name is cleared. Harris ultimately chooses to take Mogo back to his home in Africa to set him free.