My landlord has verbally agreed to let me keep a dog. Now, after I have my dog already 6 months, I was forbidden in writing to keep a dog in this property. Is this allowed?

This is where customary law usually comes into play. If you as a tenant have owned a dog for a long time, you may keep it. This is in any case the case if the landlord or the property manager knew about your dog. In such a case, it does not matter what is written in the rental agreement or whether the landlord has expressly permitted the keeping of the dog. The animal is considered to be tacitly approved.

BUT: This permit may be revoked if the dog gives rise to complaints.

Art. 257 f OR. states that the tenant must use the thing with care and show consideration for housemates and neighbors. This means that the landlord does not have to tolerate emissions from keeping animals if they have an unacceptable effect on the apartment and the house, the fellow tenants and also on neighboring properties.
It makes sense for a tenant to get consent in writing. Once the (written) consent has been given, it is difficult for the landlord to defend himself against the unreasonable nuisance of keeping dogs. Important reasons must be presented for the withdrawal of the consent once it has been given.

Often landlords do not allow pets to be kept because rights and obligations are not precisely regulated anywhere. This can be remedied by a contract addendum of the IEMT (www.iemt.ch), which has developed rules that take into account the interests of both landlords and tenants as well as the animals' need for species-appropriate husbandry. Suggest this contract addendum to your landlord.