Yes, Dionysus' one form is a bull. he is also called 'Bromios' (the roaring one). It is ironic that Pentheus sees Dionysus as a bull (epiphany) when Dionysus takes over his mind and makes him actually mad as opposed to before, when he was sane and Tiresias calls him mad for not seeing Dionysus as a God.
Also, try to see the difference between the effeminate stranger as Dionysus and Dionysus' true form as a bull. It shows the two sides (duality) to Dionysus and is a big theme in the play. The stranger is calm, gentle and rational but Dionysus as a God is violent as he shakes down Pentheus' palace. The stranger is extremely self-controlled and calm, which is ironic as his followers, the Maenads are ecstatic and have let go of their rationalism. This is the power Dionysus has over pthe character's minds. Pentheus is basically Dionysus' foil and his exact opponent. While he is angry and agitated, Dionysus is calm and self confident. Dionysus' is "hunted" by Pentheus at the beginning (fawn simile) but the roles are reversed as the hunter becomes the hunted. This power is also a big theme in the play. Pentheus tries to enforce his power on Dionysus but Dionysus is a god and cannot be controlled, which angers Pentheus. So one could argue, that although Dionysus was " hunted" at the start and although, there is a see-saw of power, Pentheus could never harm him as Dionysus is a god and is almost the author of the play. He dictates and orchestrates the play.
There are tons more points but it'll take too long to write them. lol. Hope this helps