In crypto trading jargon, dino alts or dinosaur altcoins stand for older, more established altcoins.
Older and more established altcoins are Ethereum, Litecoin, Monero and such. The reason for separating them when talking about altcoin markets is that the dinosaur cryptos usually move differently from the newer cryptos.
For instance, DeFi altcoins which are newer altcoins have moved completely out of joint since their inception.
On the contrary, Litecoin and to some extent Ethereum fiat markets move in a more correlated way to bitcoin markets.
It is useful for market analysis to split different assets based on fundamental as well as perceived value.