We have markers in the backyard for the one dog and 6 cats who have gone to the bridge since we moved here 16 years ago.
The first few I ordered from Alsto - marble bricks with their names and dates. The last two I have purchased from a local monument store. I just take a brick of my choice in, tell them what to engrave, and they have come out beautifully. This place also started a pet crematorioum in the past few years.
When we buried the last few, who were cremated, I put the ashes in a tupperware container, with a laminated photo and a copy of the Rainbow Bridge poem.