Endless debate in the Western Pokemon community exists over whether these three are the "Legendary Dogs" or "Legendary Cats", and with no real formal title as a trio in the localized versions the debate will likely continue indefinitely, especially with their recent return to the spotlight via their Paradox form debuts in Scarlet/Violet
. In Japan though my understanding is that they're commonly known as the "三聖獣" - lit. Three Sacred Beasts, somewhat similar to the grouping of the "Four Lords" lifted from Chinese myth iconography (Their Japanese names, if you're familiar, are Seiryu, Byakko, Suzaku and Genbu)
. I love the Sacred Beasts Entei, Raikou and Suicune as emblematic of an earlier era for Pokemon when the world at large had much more of an indistinct "Mythic" vibe - while modern Legendaries are often ascribed some kind of cosmological significance the description of the Sacred Beasts as embodying the lightning, fire and rain that destroyed one specific tower built in the Johto region feels much more true to the idiosyncrasies of genuine myth, which is rarely so neatly cut as to identify universal stewards of time, space etc.
I also love how genuinely unique their appearance is, a melange of mammalian, reptilian and even avian traits that earns them the status of representing "Beasts" in the abstract. Raikou's colourscheme especially captivates me, and this piece began as a warmup doodle of his face alone and rapidly escalated.