Lilac/Lavender Streaked, or Lightly Grey Streaked. Since he looks like the April '09 grey streaked rat.
A lot of recent litters look to be based on specific things, so maybe some digging into old discussion threads to see what they said about their designs for, for example, the September 2018 rats that look to be based on some species of animal. I just can't tell what it is. xD
October 2019: Golden Leaf Litter Collared Golden Leaf Rat || Countershaded Golden Leaf Rat || Ombré Golden Leaf Rat Golden Leaf PPS Pinkie
Halloween 2019 Gold Snaketail Rat || Green Snaketail Rat || Purple Snaketail Rat Purple Lantern Rat Shaggy Ink Cap PPS Pinkie || Glowing Mycena PPS Pinkie || Bleeding Mycena PPS Pinkie Werewere-kōkako PPS Pinkie || Bird's Nest Fungus PPS Pinkie
November 2019: Corn Litter Dull Corn Rat || Dark Corn Rat || Vivid Corn Rat
Christmas 2019 Orange/Skeleton Candle GUPPS Rat
January 2020: Pinto Leopard Litter Teal Pinto Leopard Rat || Yellow Pinto Leopard Rat || Orange Pinto Leopard Rat Blue Pinto Leopard Rat || Green Pinto Leopard Rat || Red Pinto Leopard Rat Purple Pinto Leopard Rat
Chinese New Year 2020 White CNY Rat || Yellow CNY Rat || Zhū CNY Rat Pink CNY Rat || Flower CNY Rat
Summer Event 2020 - Carnival Black Balloon Rat || Grey Balloon Rat || Brown Balloon Rat Blue Carnival Parade Rat
October 2020: Pumpkin Litter Pumpkin Pepitas Squash Rat || Bicolor Pear Pumpkin Rat || Kabocha Squash Rat Sweet Dumplig Squash Rat || Pumpkin Squash PPS Pinkie
Christmas 2020 White Gift-wrap Rat || Yellow Gift-wrap Rat || Candy-striped Gift-wrap Rat Blue Gift-wrap Rat || Snow Gift-wrap Rat || Green-striped Gift-wrap Rat
February 2021: Squirrel Litter Prevost's Squirrel Rat || Variegated Squirrel Rat
Grey Petoskey Stone Rat | Dry Petoskey Stone Rat | Wet Petoskey Stone Rat Mexican Tetra PPS Pinkie | Waterfall Climbing Cave Fish PPS Pinkie
Halloween 2021
Hairless Monster-under-bed Rat | Toxic Monster-under-bed Rat
November 2021
Chocolate Veriberk Rat | Chocolate Siamese Rat
Christmas 2021
Snowman Rat | Pohutukawa Rat
Christmas 2021: White Elephant Party Litter
White Elephant Party Rat | Sock White Elephant Party Rat | Sweater White Elephant Party Rat Jingle Bell Collar White Elephant Party | Candle White Elephant Party Rat
'Tea' Rae Dunn Rat | "& Mice" Rae Dunn Rat | 'Laugh Live Love' Rae Dunn Rat 'Nibble' Rae Dunn Rat | 'Cheese Lover' Rae Dunn Rat | 'Spice' Rae Dunn Rat 'Coffee' Rae Dunn Rat | 'Sugar' Rae Dunn Rat | 'Squeak' Rae Dunn Rat