All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You should ask Allah for forgiveness and repent to Him in all circumstances; for a Muslim is enjoined to repent constantly. Allah, The Exalted, Says (what means): {And turn to Allah in repentance, all of you, O believers, that you might succeed.} [Quran 24:31] Ibn Taymiyyah said: "A person is commanded to always repent to Allah, The Exalted." [End of quote]
Assuming that what you did was a sin, it is sufficient for you to seek forgiveness of Allah and repent of it. Seeking the forgiveness of animals has no basis in the Sharee‘ah! As far as the forbidden aggression against animals is concerned, repentance between the doer and his Lord is sufficient.
Bareeqah Mahmoodiyyah reads: “An example of the sins that are kept between a sinner and Allah, The Exalted, is a transgression against animal rights, such as by way of bestiality, killing, beating without a valid excuse, hitting its face even with an excuse, riding and overloading it beyond its capacity, and denying it fodder and water.
This category is very problematic because such aggression is only expiated by sincere repentance, imploring Allah, and weeping (out of remorse).” [End of quote]
Allah Knows best.