Replica Youkai
In Episode 12, Dr. Yugami Hagase created replicas of Youkai monsters from the children Tengu kidnapped.

Kappa
Also came back in Episodes 39 and 53.
Oboroguruma
Also came back in Episodes 39 and 52.

Azuikiarai
Also came back in Episodes 34, 39 and 53.
Nurikabe
Also came back in Episodes 34 and 39.

Mokumokuren
Also came back in Episodes 39 and 53.