[First, a word from our hosts...]
Bismillahi Al-Rahmani Al-Rahim
Is it permissible to neuter animals?
The issue of castrating animals male sheep is explicitly mentioned in books. The base ruling for castration is that it is unlawful because it contains an amount of cruelty. But in cases where the sheep will be eaten and there is some benefit in castration, it becomes permissible. If the sheep is not intended for food, then there is no legal justification for doing so.
As for other animals, the obvious conclusion is that it is unlawful since there is no legitimate reason to overrule the base ruling.
And Allah knows best.
Wa al-salamu `alaykum,