While intelligence is not strictly defined, we have a vague, collective definition. Science writers regularly produce rankings of intelligent species. Wikipedia also keeps inventories of cognitive features for well-known species. For example, elephants exhibit pointing, mimicry, and self-awareness. These social lists, when merged, resemble a hierarchy where the species with the most cognitive features number the least, whereas the species with the least cognitive features number the most.
Tier 1 (Generalist): humans
Tier 2 (Clever): chimpanzees, elephants, octopus, dolphins, dogs
Tier 3 (Smart): pigs, monkeys, squids
Tier 4: crows, parrots, squirrels, et. al
Tier 5 (Specialist): spiders, et. al.
Note: without human intervention, wolves (which evolved into dogs) would likely be Tier 3.
Timescales
Humans diverged from chimpanzees 6 ma (million years ago).
The biggest group of what we consider smart animals are smart mammals, whose common ancestor are placental mammals (90 ma).
There are no marsupials (87 ma) in tiers 4 and above. Marsupials branched out from Therian mammals (160 ma). Therians give birth without using a shelled egg.
Octopi and humans share a common ancestor of Nephrozoa (560 ma), which all have bilateral symmetry. There are no intelligent species outside of Nephrozoa.
