Of course you can bless animals just like you can bless a home or anything else. There is a lot in the power of prayer!
Honestly, I'm in agreement with most who have commented in this thread, with some great quotes and examples. Especially when requested by the owner, though even in general if one feels it may be helpful or am prompted by the Spirit, there shouldn't be any reason why a Priesthood holder couldn't given animals a blessing I feel.
Whether farm animals or pets, we as humans have often shown great attachment and care for the animals under our care. So even if the animals cannot have faith for themselves, we can still have faith enough to have such blessings performed.