In a bowl, mix shredded chicken, cream cheese, cheddar, ranch seasoning, and green onions until combined.
Separate crescent dough into triangles and press seams to seal into rectangles.
Spoon about 2 tablespoons filling along the short edge of each rectangle, then roll tightly into taquitos.
Lightly oil the slow cooker bottom and arrange taquitos seam-side down in a single layer; work in batches if needed.
Cook on LOW for 1–1½ hours until heated through and dough is set.
If you want browned edges, transfer taquitos to a baking sheet and broil 1–2 minutes, watching closely.