A triolet is a short poem that has a fixed form, taken from French poetry, and is close to the rondeau, a form that emphasizes repetition and rhyme. Composing the triolet there are three stanzas and eight lines that use two rhymes--ab aa abab. The first line is repeated as the fourth and seventh lines and the second line is repeated as the eighth. Each line is usually written in iambic tetrameter, meaning an unstressed syllable is followed by a stressed four times [four feet] in each line. Here is an example of a triolet written by the Victorian poet Thomas Hardy,
"Birds At Winter"Around the house the flakes fly faster,And all the berries now are goneFrom holly and cotoneasterAround the house. The flakes fly! – fasterShutting indoors the crumb-outcasterWe used to see upon the lawnAround the house. The Flakes fly fasterAnd all the berries now are gone!
Now, to write a first line for your triolet about dogs, you may wish to make some type of statement or description. For example,
A dog is better than alone [meaning to be alone, but utilizing the iambic tetrameter]