Legal rights to dogs, cats and donkeys too are dependent on various factors. If upon receiving the donkeys you entered into a written agreement, courts would consider the terms of the agreement, but would also look at compliance. For example, if under the terms of the contract, you were to be paid for the donkey’s care but received no payment, or if the donkeys were left with you for a specified time period but that time period is long gone, courts may consider that evidence when determining who now owns the donkeys. Courts may consider verbal agreements as well, but usually when there is a custody dispute, the parties have different recollections of the agreement.
If there are allegations of cruelty, that too could sway a court depending upon the evidence presented. Sometimes when people are not really bonded with animals -- which may be the case here since you claim the woman left the donkeys with you five years ago -- these matters can be resolved by reaching a fair monetary settlement. I hope you all consider what is best for the donkeys.